Why can't I create a CNAME record for the zone name itself?

A CNAME-record cannot exist with any other records for the same name, because a name cannot both be an alias (CNAME) and something else at the same time.

From RFC1912 section 2.4:

"A CNAME record is not allowed to coexist with any other data."

If you try to create a CNAME-record for the name of the zone itself, you will always encounter this conflict because a zone always has at least a SOA-record and typically one or more NS-records with the zone name:

Instead of the CNAME-record, you can setup an A-record pointing directly to the IP address of whatever server the aliased name is pointing to.

Or you can use an ALIAS-record which is similar to CNAME but doesn't have the problem mentioned above.