Unable to enable disabled safari extentions

Restart your Mac.

Safari > Preferences > Extensions

Open the Extensions pane.

Click Search to open the Search pane.

Back to Extensions pane and try enabling it.

Enable Develop menu. Click Develop in the Safari menu bar and click "Empty Caches" twice.

Try enabling Extensions again.

https://support.apple.com/guide/safari/use-the-developer-tools-in-the-develop-menu-sfri20948/mac

I could instal and uninstall Grammarly for Safari.

Here are the steps I took.

Installing Safari Extensions

Safari > Safari Extensions > Grammarly for Safari

Downloaded it.

Launched Launchpad and clicked it.

Opened Safari Extensions and enabled it. Safari > Preferences > Extensions

I didn't want to sign in with neither Facebook nor Google.

Uninstalling safari Extensions

Disabled Grammarly for Safari. Safari > Preferences >Extensions

Clicked Grammarly extension icon in the Dock and quit Grammarly for Safari from app menu bar.

Removed Grammarly for Safari icon from Dock and Safari Toolbar.

Safari > Preferences > Extensions > Grammarly for Safari > Uninstall

Uninstalled it by selecting it from Finder > Applications and "Move to Trash" after authenticating.

See if anything in this discussion helps.

Can't tick, toggle or modify Safari e… - Apple Community

This is a legitimate bug I have been having as well. Yes restarting helps but I frequently run into this and it's annoying

I hope Apple looks into this