It's bad enough that Coinbase holds your Bitcoin for several days after you purchased it, which they should have no absolutely no right to do, but it's downright enraging when you can't even send it when they claim that you'll be able to.
I don't think I've come across a case where a particular user is restricted from making transactions with no reasons giving. But they do have the right to hold your Bitcoins as you agreed to their Terms of Service.
Not your keys; Not your coins.
They essentially let you use your Bitcoin as you do not have custody of it.
What do I do now to withdraw my remaining Bitcoin?
Only coinbase can allow you.
Have you tried contacting their support and maybe calling them out on social media?
Try the first option and wait for a reply and if that's not a satisfactory one, then you can call them out publicly.