What is the most likely reason for this?
The source DB instance has to be converted to Single-AZ first to create a read replica from it.
Enhanced Monitoring is not enabled on the source DB instance.
The minor MySQL version in the source DB instance does not support read replicas.
Automated backups are not enabled on the source DB instance.
Explanations:
The source DB instance does not need to be converted to Single-AZ to create a read replica. Multi-AZ instances can have read replicas as long as other prerequisites are met.
Enhanced Monitoring is not a requirement for creating a read replica. This feature is related to performance metrics, not the ability to create replicas.
The minor MySQL version in the source DB instance must support read replicas, but most versions of MySQL in RDS do. The issue described is likely not related to MySQL version compatibility.
Automated backups must be enabled on the source DB instance to create a read replica. Without backups, the read replica cannot be created, as they are required for replication.
From what I’ve heard, the answer is:
Automated backups are not enabled on the source DB instance.