Can't enable safari extensions

I was saying that because, the first time I only cleaned the cache and nothing happened, when I restarted nothing happened, when I cleaned the cache and immediately restart it worked.

I was having exactly the same problem with all extensions I had, after updating to macOS10.15.4

Turns out moving the window away also worked for me lol

amazing Easter egg with moving preferences window to different area on the screen.... it worked... wtf

I moved the window around and then I was able to click the checkbox next to the extensions.

Try this: add Safari to the System/Security & Privacy/Full Disk Access/Privacy preference pane. Reboot, launch Safari and see if you can activate the extensions.

This is a test.

Open Extensions pane of Safari Preferences.

Try enabling an extension.

It may not be successful.

Open Search pane of Safari Preferences.

Back to Extension pane and try enabling it.

Try "Empty Caches" and "Remove All Website Data".

Empty Caches

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

Remove website data

https://support.apple.com/guide/safari/manage-cookies-and-website-data-sfri11471/mac

Finally, after updating yesterday, Safari can activate extensions again. I tried all the cleaning cache stuff, and it never worked for my Mac for some reason, but updating did. Good luck to anyone who still has this problem.

You've install what many of those here consider akin to malware, CleanMyMac4. It's totally unnecessary and is known to have seriously compromised system and app performance. Uninstall it according to the developer's instructions. When removed rerun Etrecheck to verify all CMM files have been removed. Test to see if you can enable extensions. If not continue below.

Then boot into Safe Mode (Use Safe Mode to isolate issues with your Mac and Playing Safe: what does Safe Mode Do?) and see if you can enable Safari extensions from there. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing. 

yesterday I solved this way:

clean the cache

quit safari

open safari

try again

I found this online it worked for me.

can you go to Safari > Preferences > Extensions, then navigate to a differentpane (e.g., Advanced), and then back to Extensions? With any luck, your checkboxes might be enable-able now. 

I hope it works for you. Good luck

Empty Caches seemed to do the trick. I was able to activate the extensions.

Do you have the Develop menu enabled in Safari?

If so make sure you don't have this checked in the menu:

Have you run any "cleaning", "optimizing", "speed-up" or anti-virus apps on your Mac?

Wow isn’t this a doozy of a bug. I assumed it was just because I was using (ahem) “an unofficial Mac” while waiting for my real one’s keyboard to be replaced.

Absolutely none of the workarounds worked for me. Not safe mode, not moving the window around, developer menus, switching tabs.

But because I’m using an “unofficial” Mac, when I tried completely reinstalling the OS, and hadn’t installed graphics drivers yet (so it was in basic 1024x768 VESA mode with no acceleration) and only then when I moved the window to the top, I was able to check the box.

After installing the graphics drivers and turning the extension off, it was no longer possible to turn it on again without another reformat.