Restoring private key from old bitcoin wallet

I have old bitcoin wallet with some (not very huge) amount (last transaction was on 2014). It was created in Multibit (Classic, I guess, then I had old computer with old OS). Now I have new device, system and wallet, and want to restore old [wallet] to send bitcoin to the new one. I know, I need a private key, so I imported old wallet (it’s watch-only wallet right now) in Bitcoin Core with rescan and next try to do like this guy ->, however, after putting “dumpprivkey”-command in console, it says “the private key for this address is not known”.

That’s what I don’t understand (interesting, though), b’cause I remember I’ve backuped it and hold those files for all those years on my USB stick, but now they’re completely unusefull. 🙁

How can I resolve this problem? Anyone knows?