Issue creating an iCloud email address to associate with existing Apple ID

Hello,

I am attempting to change my Apple ID from my @gmail account to an @iCloud account, but am having issues creating the @iCloud address.

Using my MacBook I am currently trying:

System Preferences > Apple ID > iCloud > Mail > Choose iCloud email address

This gives me an option to create a new iCloud email address, however choosing an address and clicking "OK" seems to load for a second and then do nothing. I get an error if the email address is already taken, but choosing a unique address doesn't seem to do anything.

I'm not sure if this is an issue on my end or Apple's. There is a message that says "You cannot change your iCloud email address after creating it," but I assume this means I can only create one iCloud email address per Apple ID, not that I can't change the email address associated with my Apple ID.

Unfortunately I don't have any other Apple devices to test this with.

Thanks for any help that anyone can offer me :)