Can't add Mail accounts, stuck 'selecting apps'?

This just says you want to get email from this account in Apple Mail.app.

Select Next and your account should be setup. You can verify by looking under Window in the Menu bar while in Mail. Select Connection Doctor. You should see two green symbols one for incoming and one for outgoing.

My recommendations for free disk space.

The 10% free is not a good gauge because it varies so widely by drive size. I like to think of it like this....

When your disk space falls below 10GB it's like the yellow light that comes on in your car to tell you it's time to fill up.

Not only do you need space to add additional items like messages, the system needs space to run. Think of it like this. Someone sits on your chest. You can't expand your lungs to bring in more air. Even though you are in a room with air, you will suffocate. The system needs room to operate.

Easy way to monitor free disk space is to turn on Status Bar in a Finder Window. Click in a Finder Window. Under View in the Menu bar, select Show Status Bar. I suggest turning on Path Bar as well.

Often users have large files that are no longer needed like old iPhone and iPad backups. Open iTunes and open Preferences > Devices. Check the backups on your drive.

Empty iPhoto Trash. Control (right) clicking on iPhoto’s Trash icon and choosing Empty Trash from the contextual menu.

Empty the Trash. You might find lots of old Fash installers.

Outlook for Mac users often have multiple copies of old Identities. Delete the old Identities.

OmniDiskSweeper is a free app that will help you find large files.

http://www.omnigroup.com/products/omnidisksweeper/

Free up storage space on your Mac - Apple Support

Try adding the account in the Safe Mode or another admin account.

Safe Mode - About El Capitan
also Sierra

Safe Mode Startup – El Capitan. also Sierra

You mentioned your hard drive being full in your first post. Did you ever create more disk space before continuing in Mail?

Disk Space - Free Up

Disk Space – Free Up (2)

Disk Space - Free Up - Mavericks

Disk Space Filling Up – OmniDiskSweeper 10.8+

Disk Space Filling Up - WhatSize

Quit Mail.

Remove the account in System Preferences > Internet Accounts.

Uncheck Mail for any other accounts in Internet Account.

Go to your User's Library/Mail folder.

Rename Mail to Mailxxxx

Go to the Containers folder in your User's Library folder and delete com.apple.mail

Log out under the Apple in the Menu bar.

Log in.

Go to System Preferences > Internet Accounts

Add your account back. Hopefully you will get past the previous hang. Hold on adding any other accounts at this time.

Open Mail

Under Window in the Menu bar select Connection Doctor. Do you see green for both incoming and outgoing for your account.

Next check in your User's Library to see if you have a new Mail folder. We can delete the Mailxxx folder after importing any "On My Mac" local folders.

Add other accounts back by checking Mail for each account in System Preferences > Internet Accounts. I would do one at a time. If there is a problem it's easier to identify the account that is causing problems.

Show User's Library

The User's Library folder is hidden by default. To unhide:

Option 1) Select the Finder in the Dock. Under Go in the Menu bar > hold down the Option key and you’ll see the Library.

Option 2) You can also select the Home folder in the sidebar of a Finder window.

Pull down the “View” menu and select “Show View Options”

Check the box for “Show Library Folder”