Cannot log into iCloud on mbp.

I get an error message when I try to enter my Apple Id password on my MacBook Pro. Thus I am unable to log into iCloud/iTunes and cannot update the software or download additional apps to my computer. I have tried trouble shooting, made sure time/date accurate etc, have reset my password, logged out on all other devices. Made sure updated on all other devices. Apple support doesn’t seem to be able to guide me thru effectively resolving the problem either. Still can not get into my iCloud on my mbp. Please help.