I am unable to sign in to iCloud on my MacBook Pro

I have an iPad Pro and a MacBook Pro, and I can't sign into iCloud. I can sign in to the Apple website, there seems to be nothing wrong with my account whatsoever, credentials are correct and confirmed, but when I try to log in to iCloud on my iPad I get the error "Your Apple Account could not be used to sign in to iCloud". On my MacBook, it asks me to accept terms and conditions and then a window pops up saying 'ACCOUNT_UNAVAILABLE" with an 'OK' button.

I think this is preventing me from accessing the developer program, which I paid for five days ago but can't use and I've sent two emails to Apple now, with no response. This is now affecting my work, so I need a resolution.

Does anyone have any insight or possible solution?

[Re-Titled by Moderator]