Hi Look like you are trying to delete default database. A postgres database is created under your server. The postgres database is a default database that's meant for use by users, utilities, and third-party applications. (The other default database is azure_maintenance. Its function is to separate the managed service processes from user actions). You can't access this database and it seems Deleting the default PostgreSQL database is not supported in Azure Database for PostgreSQL flexible server. You can only delete user-created databases.
Hope this help!
Thank You!