Tampermonkey

macOS Mojave/High Sierra

Safari 12

A user has to download the Safari App Extension from App Store.

Otherwise installation will fail.

For info:  https://developer.apple.com/documentation/safariservices/safari_app_extensions

If you have other extensions installed, have you tried disabling them, and then tried to install Tampermonkey? Are you running any anti-virus programs?

Try running this program in your normal user account, then copy and paste the output in a reply. The program was created by etresoft, a frequent contributor.  Please use copy and paste as screen shots can be hard to read. Click “Share Report” button in the toolbar, select “Copy Report” and then paste into a reply. This will show what is running on your computer. No personal information is shown. If the log won’t post, try posting it in Pastebin and provide a link in a reply        Pastebin

Etrecheck – System Information    10.10 and later

Click the link I posted.

Towards middle of the page, on the right

side there is search box.

Enter Tampermonkey and hit return.

That should return Tapermonkey

with an install now button. Click it.

In my earlier post, I posted a screenshot of it.

What happens when you try? Where are you trying to download it from?

https://safari-extensions.apple.com/?q=tampermonkey

I don't see any software that normally causes problems. Did you try disabling the 2 Safari extensions?

Did you try the download link I posted?

See if it will install in the Safe Mode.

Safe Mode - About

There was no problem with downloading and installing Tampermonkey.

Try App store again.

https://safari-extensions.apple.com/