The following resource(s) failed to create: [AWSEBRDSDBSubnetGroup].

Hello.

Are you selecting and creating subnets in multiple Availability Zones?
RDS subnet groups must be created by selecting subnets that use at least two Availability Zones.
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_VPC.WorkingWithRDSInstanceinaVPC.html#USER_VPC.Subnets

Each DB subnet group should have subnets in at least two Availability Zones in a given AWS Region. When creating a DB instance in a VPC, you choose a DB subnet group for it. From the DB subnet group, Amazon RDS chooses a subnet and an IP address within that subnet to associate with the DB instance. The DB uses the Availability Zone that contains the subnet.

Also, can you check if there are any other errors occurring?
For example, it is possible that the AWS resource cannot be created due to a permission error.