Can't enable indexing

Restored a OS backup image to a backup Win 10 PC. Ran the troubleshooter.

Seems your problem isn't with search and indexing as such, but something running or some other problem that is preventing proper access to the device in question.

(Error 21)

I.e. that's the problem you need to fix- whether that be some 3rd party program, a software conflict, permissions or some hardware-related issue. Impossible to be precise- numerous possible causes.
Suggest you research error 21 for more.

Trying to fix 'search' as such doesn't look like the way forward.