After doing a lot things and wasting two days I got following method. First in my subscription user name is visible so use following command to change user.
grant {your_user_name} role to current_user
Now connect to that DB and Alter your subscription but instead of Enable you have to delete and recreate it, but remember you must have master credentials. This work and these all are commands of postgresql and I got help from its documentation, I don't know why it aws rds postgres is not postgresql.
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