Why can't I set a PS4 acc as primary?

My friend would like to share his ps + membership with me. He gave me his account data and when I try to activate this ps4 as primary in the settings it says that I have already activated another ps4 as primary and that I have to deactivate it first (however, I did not set ps4 to primary)

You can have multiple accounts on primary that is not the problem. He just has to first deactivate on the account he gave you on his PS4 before you can activate it.