From my understanding you are trying to migrate your snapshot for an RDS PostgreSQL database but the option is greyed out/disabled. It could be because of a compatibility issue. You would just need to ensure that your RDS PostgreSQL instance is compatible with the requirements of the database you are migrating to.
In the case you are migrating to an Aurora PostgreSQL cluster, you can migrate RDS for PostgreSQL snapshots that are compatible with the Aurora PostgreSQL versions available in the given AWS Region.
For example, a snapshot from an RDS for PostgreSQL 11.1 DB instance can be migrated to Aurora PostgreSQL version 11.4, 11.7, 11.8, or 11.9 in the US West (N. California) Region. An RDS PostgreSQL 10.11 snapshot can be migrated to Aurora PostgreSQL 10.11, 10.12, 10.13, and 10.14. In other words, the RDS for PostgreSQL snapshot must use the same or a lower minor version as the Aurora PostgreSQL.
You can find out more information here in the official AWS docs: https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/AuroraPostgreSQL.Migrating.html
You can refer to this past forum if need be where a similar question was answered: https://forums.aws.amazon.com/thread.jspa?threadID=311992
You can also refer to this link, however be careful as this is not an official AWS link: https://stackoverflow.com/questions/67749555/unable-to-migrate-rds-postgres-to-aws-aurora
I hope this helps.
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