necessary permissions?

My new iMac is playing up after a recent update (Catalina 10.15.5) and now when I want to move a file or folder I get an error warning that I 'don't have the necessary permissions' or 'Finder wants to move xxx enter your password'.

Can someone tell me what or where the 'necessary permissions' now are? They weren't necessary a few days ago, so can't be THAT necessary.

And the same with permissions. I've asked myself and have given myself permission, but who else do I have to get permission from? That's like calling the guy down at the dealership and asking him for permission to drive my car to the shops, or if I can move that shopping bag onto the back seat.

I've set all the folders in the Home folder to read and write 'me only'.. if that affects things?

Any info gratefully received. Thx