Creating an iCloud email
Lots of reasons why Apple doesn’t allow mail creation. Best bet is to ask Apple Support about your case specifically:


https://support.apple.com/contact


Sometimes it’s because too many accounts were created from that device or location, sometimes it’s a conflict reserving the alias or address, sometimes it’s just that millions of requests across thousands of computers get stuck on the server side or break. 

Support will know is the system is up or down or if you hit a specific roadblock.