Problem Description:

Usually we face this issue while dropping user from database which owns database roles and error code 15421 indicates issue of database role ownership which need to be modified.

Solution:

Run below query on database to check database roles owned by that user which need to be removed.

select dp2.name,dp1.name from sys.database_principals as dp1 join sys.database_principals as dp2 on dp1.owning_principal_id=dp2.principal_id where dp1.type='R' and dp2.name='<username>'

get the role name from above query and pass it to below query.

use <dbname>
go
alter authorization on role::[rolename] to [dbo]
go

once rolename authorization is changed to dbo, you can now drop the user from the database.