Apps using "postgres" user instead of my_user in the database

My postgres databse keeps the value postgres for database name even when I create new db and new user. It seems that no matter what I do, I cannot alter it.facing the error I want to use my database name and user when i change the database name to “postgres” and username into “doadmin” application connecting to the database else showing this error on change the name of my db and user

apps@feedback-app-7-jx2mf:~$ rails db:migrate
 rails aborted!
 ActiveRecord::StatementInvalid: PG:: Insufficient Privilege: ERROR: permission denied for table schema_migrations /workspace/bin/rails:5:in `<main>'
Caused by:
 PG:: InsufficientPrivilege: ERROR: permission denied for table schema_migrations
 /workspace/bin/rails:5:in `<main>'
 Tasks: TOP => db:migrate
 (See full trace by running task with --trace) apps@feedback-app-7-jx2mf:~$