I had the same issue. I think you may need to wait until the time you claimed it.
For example, if you claimed it at 1:00pm on November 2nd, you'd have to wait until 1:00pm on December 2nd to claim it again.
I'm not 100%, but this worked for me. If you're still having this issue, you should contact one of the higher-up staff members, or submit a ticket. If you're in the ManaCube discord, there's a section for issues/help in which you can report it, if the problem persists.
If you can remember what time, or roughly when you claimed your rewards the first time, wait until then, if you can't remember, I'd leave it till a while later, or get in contact with a staff member if you're sure you should be able to claim it. (You can check when you last claimed your monthly rewards here:
https://store.manacube.com/status). Hopefully this gets resolved soon!