You can't use Aurora MySQL replicas as a source for AWS DMS unless your DMS migration task mode is Migrate existing data—full load only.

for change data capture to work AWS DMS would require to read binlogs which holds information about transactions. With Aurora MySQL reader endpoint Binlogs are not accessible thus cdc wont work.

DMS uses binlogs in MySQL for CDC. In Aurora MySQL, the readers do not use binlogs for replication replication. [+] https://docs.aws.amazon.com/dms/latest/userguide/CHAP_Source.MySQL.html#CHAP_Source.MySQL.Limitations

Since aurora has a shared volume which is shared by the all the readers and the writer, the changes are directly made to the volume. Reader then reads the data directly from the volume. [+] https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/Aurora.Overview.html

Since reader instances in aurora do not have any binlogs as the write queries never reach the reader instance, CDC task will not be able to replicate the ongoing changes from the reader instance to your redshift database.

As an alternative, you can choose to create an Aurora read replica for your cluster and then use the writer instance of the replica cluster for the CDC task. Since RDS MySQL read replicas and the Aurora read replicas uses binlog for replicating changes from source cluster/instance, you should be able to setup CDC task from read replicas.

"If you are using an Amazon RDS MySQL or Amazon RDS MariaDB read replica as a source, enable backups on the read replica, and ensure the log_slave_updates parameter is set to TRUE." [+] https://docs.aws.amazon.com/dms/latest/userguide/CHAP_Source.MySQL.html#CHAP_Source.MySQL.AmazonManaged