Issue :- The server principal owns one or more endpoint(s) and cannot be dropped hence unable to delete a login from SQL Server
How to Fix.
Find out the information of granter and grantee principal id along with the granted permission
You can see from the below screen the account is having authorization on Endpoint, to confirm this you can fire a select query on sys.enpoints or you can use below mentioned query to fetch details of account who owns endpoint.
SELECT p.name, e.* FROM sys.endpoints e
inner join sys.server_principals p on e.principal_id = p.principal_id
The server is configured is having mirroring configured on it and this account is owning that one single mirroring end point, so we cannot delete his account who is earlier SYSADMIN and owning this endpoint.
- Change the authorization on endpoint
Alter Authorization on endpoint::Mirroring to [SA] \[domain\account]
This will change the authorization of endpoint without disturbing the mirroring and then you can drop that Login.