Safari can't install this extension

Where are trying to install it from?

Are you trying to install the extension by going Safari/Safari Extensions? If not, try there.

Please back up all data before proceeding.

Launch the Keychain Access application in any one of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad and start typing the name.

Use the search box in the toolbar of the Keychain Access window to search for an item named "Safari Extensions List". If found, delete it. Quit Keychain Access.

Quit and relaunch Safari.

When you select the item and press the delete key, you should be prompted to confirm. Does that happen?

GEnt --

Are you using your Administrator account on your Mac?

Please post a screenshot that shows what you're doing. Be careful not to include any private information.

Start a reply to this message. Drag the image file into the editing window to upload it. You can also include text in the reply.

Whether you're an administrator or not is irrelevant.

In this case I was able to delete the extensions folder you suggested.

I absolutely did not suggest that you delete any folder. Whatever you deleted, please restore it from your backup.

If the problem is still there, try booting into the Safe Mode using your normal account. Disconnect all peripherals except those needed for the test. Shut down the computer and then power it back up after waiting 10 seconds. Immediately after hearing the startup chime, hold down the shift key and continue to hold it until the gray Apple icon and a progress bar appear. The boot up is significantly slower than normal. This will reset some caches, forces a directory check, and disables all startup and login items, among other things. When you reboot normally, the initial reboot may be slower than normal. If the system operates normally, there may be 3rd party applications which are causing a problem. Try deleting/disabling the third party applications after a restart by using the application un-installer. For each disable/delete, you will need to restart if you don't do them all at once.

Safe Mode - About El Capitan

Safe Mode Startup – El Capitan.

See this post and associated discussion:

Re: Safari 9 - I cannot install extensions

The symptom is the grey padlock icon on the address bar of safari when you go to https://safari-extensions.apple.com. The cause for this is a bad certificate. To solve this issue:

1. open safari and go tohttps://safari-extensions.apple.com
2. switch to safari and click on the grey padlock on the address bar. Be sure to click exactly on the padlock icon, otherwise it just selects the URL for edit.
3. a window drops down that looks like this:
4. 
5. click on "Show Certificate"
6. The details of the certificate appear:
7. 
8. note that the certificate shown above is valid, so you should see something different, specially where it says "This certificate isvalid". Crucially, this window identifies the certificate you need to delete in keychain. In this case it's called 'VeriSign Class 3 Public Primary Certification Authority - G5'
9. open keychain (as described by Linc above)
10. choose certificates on the right
11. locate the relevant certificate and delete it with the backspace key. They are ordered alphabetically, so it should not be difficult. If you can't find it, try different keychains on the top left corner:
12. 
13. switch to safari and reload https://safari-extensions.apple.com/, the padlock icon should turn nice and green:
14. 
15. You should now be able to install lastpass directly from the website, since the button/link is visible