Server principal 'DB_A667B6_kofoglu_admin' has granted one or more permission(s). Revoke the permission(s) before dropping the server principal. (Microsoft SQL Server, Error: 15173)

Since DB_A667B6_kofoglu_admin exists only as a login at Server level, we query if DB_A667B6_kofoglu_admin has granted permissions to any other Server Principal using the following script :

Select * from sys.server_permissions
where grantor_principal_id =
(Select principal_id from sys.server_principals where name = N'DB_A667B6_kofoglu_admin')

Output

class class_desc major_id grantee_id grantor_id Type Permission_name state state_desc
101 SERVER_PRINCIPAL 4979 2 4979 VW VIEW DEFINITION G GRANT

2 was the principal_id for ‘public’
4979 was the principal_id for ‘DB_A667B6_kofoglu_admin’

How to know the above? Use the following script:

SELECT pr.principal_id, pr.name, pr.type_desc,   
    pe.state_desc, pe.permission_name   
FROM sys.server_principals AS pr   
JOIN sys.server_permissions AS pe   
    ON pe.grantee_principal_id = pr.principal_id;

find the principal_id and name from the generated table

you can verify the grantee from SSMS UI or execute the following script to revoke the permission.

REVOKE VIEW DEFINITION ON LOGIN:: DB_A667B6_kofoglu_admin FROM [public]

USE MASTER

REVOKE VIEW DEFINITION ON LOGIN:: DB_A667B6_kofoglu_admin FROM [public]

Make sure all output permission was revoked, you can now drop the login successfully.

refernece: Drop failed for Login since it has granted one or more permission(s) | Microsoft Docs sys.server_principals (Transact-SQL) - SQL Server | Microsoft Docs