Hi,
You can take AZ-400 exam without having Azure Developer (AZ-204) or Azure Administrator certification (AZ-104), however, you will not earn Microsoft Certified: DevOps Engineer Expert certification without at least one of them. You can go ahead and take AZ-400 now if you are ready and then take either AZ-204 or AZ-104 and then certification will show up.
Please click Accept Answer if the above was helpful.
Thanks.
-TP