You see that error because, RDS PostgreSQL cannot be spun up on T2 instance class.
You can only create M6g, M5, T3, R6g, and R5 instance types. - https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/CHAP_PostgreSQL.html
Change the instance class and try again. I am sure it would work.
The error message is correct. Amazon RDS does not support db.t2 family instances for PostgreSQL version 13 and higher: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Concepts.DBInstanceClass.html#Concepts.DBInstanceClass.Support
The only thing you can do to get your database instance created is to switch to a lower PostgreSQL version or change the instance type to one of the supported instance families, like t3 or t4g.
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