This post contains tips that help Fix Safari can't install this extension while trying to install a 3rd-party plug-in on your Safari for Mac. Run this pro-grade app remover@ macpaw.7eer.net/c/376211/480286/1733 to clean up unwanted/unused applications on your Mac. Learn more?
Solution 1: Delete "Safari Extensions List" from Keychain Access. Plan B: Step 1: use Safari to access safari-extensions.apple.com/ Step 2: click on the green padlock icon Step 3: click on "Show Certificate" Note that the certificate shown above is valid, so you should see something different, specially where it says "This certificate is invalid". Crucially, this window identifies the certificate you need to delete in keychain. Step 4: Run Keychain > Certificates > Delete relevant bad/invalid item from Keychain. Solution 2: Try deleting/disabling some unused/unknown third-party applications by using the recommended app remover PRO in this video guide. Solution 3: Create a new admin/root user and try again. Solution 4: Delte Extensions folder from your own original user's /Library/Safari/ directory. Relaunch Safari and try again. Other options are: 1. install the extension manually. 2. Disable all of the extensions you're using and try again. 3. Try booting your Mac into the Safe Mode and try again.
To load your Mac in its Safe Mode, simply press and hold the Shift key while it boots. You can stop holding the Shift key when you see an Apple logo and progress bar.