Unable to delete SQL Server user from database, error: The database principal owns a schema in the database, and cannot be dropped.

Workaround:

Example: to delete user “testadmin” from database “TestDB”.

USE TestDB;
SELECT s.name
FROM sys.schemas s
WHERE s.principal_id = USER_ID('testadmin');

-- outcome e.g. db_denydatareader

ALTER AUTHORIZATION ON SCHEMA::db_denydatareader TO dbo;