sierra sometimes wants a PIN, not password?????

The only two cases that I can think of that a PIN might be asked for are -

1. If the computer has been locked via 'Find My Mac'
2. If you have turned on two-factor authentication on your Apple ID, see - Two-factor authentication for Apple ID - Apple Support

The later seems the most likely. If the Mac was acquired from a previous user and not fully wiped then it might also relate to the previous owners Apple ID.

For me I had a CAC reader plugged into my USB while trying to input this PIN, but as soon as I removed it from USB it went from PIN to Password and my username went from gray to black. Put in my password and all is well. Hope this helps.

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The only time I've ever seen a PIN prompt while actually logged onto a Mac is when using a Smartcard or USB Yubikey device. Or perhaps when Two-Factor Authentication kicks in such as when I've changed my Apple ID password and I'm logging on to iCloud services for the first time on this Mac. The other PIN prompt is from the Mac being locked via iCloud Find my Mac but that won't even let you boot without the PIN code and this doesn't sound like that scenario at all. The iCloud PIN Two-Factor authentication login pops up and asks for a 6 digit PIN and there are 6 little boxes for each digit. The PIN code is typically sent via SMS Text Message to your iPhone.

Sounds like the user account is messed up due to the Keychain issues as well. I would first, try to create a new local account and make it an admin then logout and back on to test. If everything works like it should, backup your data and consider reloading the OS/Apps and restoring your data. You might want to do that via a USB boot drive and downloading the latest Sierra version. In other words, perform a squeaky clean install. Backup, erase the internal disk completely and install from scratch the OS then your Applications then restore just the data you need. You might have to give up on the keychain if you cannot unlock it. Keychain Access -> Preferences -> Reset My Default Keychains.

You can still get Sierra (not High Sierra) here:

https://support.apple.com/en-us/HT208202

Burn a USB bootable installer thumb drive

How to create a bootable installer for macOS - Apple Support

I've got the same issue in my keychain access. I can't change trust settings for sites I need to access because it asks for a PIN instead of a password - I'm an admin, bought the computer new and never set a PIN.

Randomly it will ask for a password not a PIN and I can make updates but not when I seem to need it most.

I have the exact same problem. Also bought my Mac new, also never set a pin. I usually am asked for a password when I am trying to install software, but right now I am trying to install a specific program and keep getting asked for a PIN I don't have. In this dialog box my name is prefilled and grayed out, so it can not be changed and there is a second box where normally the password is entered, but this time it demands a PIN. Apple Support, please address this, this is a major problem.

I also think this is a smart card issue.

I can't remember what happens when you insert a smart card on Sierra, but I think it is the same as High Sierra.

The OS asks what you want to use the Smart Card to do. I can't remember which option causes it to use the card for Login/authentication, but one of them does. As long as the card is inserted into the card reader, the OS will ask for the Smart Card PIN when you need to authenticate instead of a password.

From what I have found, choosing either option from that dialog will make using a US Govt Comman Access Card difficult when trying to access government websites from the Mac. If asked, I just Cancel which then allows me to only use the card through software such as Centrify (as mentioned in the original post).

After a rather fruitless discussion with Apple support on twitter ("contact the software vendor") the Apple support guy recommended rebooting the machine. That didn't help either, but I decided to try a cold start (shutdown, let sit for a minute, then turn on again), and that surprisingly did help. LastPass is installed now, and next time this happens (unless Apple actually fixes it) I know to cold start the machine. Maybe this helps someone who also encounters this problem.

Pin is on back of cac reader

To correctly sell or transfer ownership of a Mac, even to someone in your family, you should follow these instructions. Until you do, you will encounter numerous and continual problems such as those you describe.

Refer to What to do before selling or giving away your Mac

1. If you enabled FileVault, disable it in System Preferences > Security & Privacy.
2. "Deauthorize" your iTunes account. Same for Audible if you have one.
3. System Preferences > iCloud > de-select "Back to My Mac" and "Find my Mac".
4. Sign out of iCloud. Select "Delete from Mac" when it appears.

Next: Remove all your personal information by completely erasing the Mac's internal storage.

1. If your Mac shipped with a grey System Install DVD, start your Mac with that disc inserted in the optical drive while holding the c key to boot from it instead of its internal volume, which should be erased before selling it.

   Apple stopped shipping Macs requiring those discs in August, 2011.

2. If your Mac did not ship with discs, boot OS X Internet Recovery:
   • using three fingers press and hold the following keys: ⌘(command), option, and R.
   • With a fourth finger press the power button to turn on the Mac.
   • Keep the other three fingers where they are until you see the "spinning globe" icon.
   • This method forces the Mac to download its originally installed OS from Apple's servers, which will not require an Apple ID to install.
3. Remove any Open Firmware password if you created one: select Firmware Password Utility from the Utilities menu and remove it.
4. Select Disk Utility from the Utilities menu.
5. Remove any partitions you may have created.
6. Select the Mac's hard disk icon, then select the "Erase" tab.
7. Select the "Security Options" button and erase the disk.
   • The more "securely" you erase the disk, the longer it will take.
   • The fastest method is sufficient since all but the most expensive techniques and equipment will be able to recover securely erased data.
8. When it finishes, quit Disk Utility.
9. Select Install Mac OS X from the Utilities menu.
   • An Apple ID will not be required. If a prompt for an Apple ID appears, return to Step 5.
   • Do not create any user accounts.
10. When it finishes, shut down the computer.
11. If you want to install the bundled apps that were included with your Mac, restart by using your Applications DVD if one was included, and install the bundled apps.

    Apps bundled with newer Macs that shipped without discs cannot be transferred. Its new owner must purchase them from the Mac App Store using his or her own Apple ID.

If the Mac is being sold to someone outside the family consider the following additional information:

1. System Install DVDs that came with your Mac should remain with it forever, and must be included with the sale.
2. Consider including your AppleCare certificate if you bought it, printed documentation, even the box if you still have it.

   AppleCare stays with the equipment and is transferable.

3. Execute a bill of sale showing the Mac's serial number.
4. Once no longer in your possession, remove the Mac from your devices in My Support Profile.

Did you ever figure this out? I also am asked for a PIN from my installer but don't ever remember setting up a PIN code for my computer.