Hey jenwei,
Thanks for reaching out, and for the detailed explanation.
From the pictures you sent, it looks like you’re in us-west-1 region.
But as far as I know, AWS App Runner is not available in US West -N. California (us-west-1).
Does the App Runner service run in a different region from the RDS instance and the Secret manger? If so, can you add REGION to the environment variable of the app runner service? the value should be the region of your secret, as it looks from the snapshots it should be us-west-1 .
Another option is to create a MySQL server and a secret in regions where App Runner is supported, for example, us-west-2.
Let us know if everything worked and if not do not be afraid to contact us again.