The login Amtpl is already mapped to a different database user. Use the following to identify the logins current database user account
use [yourdb]
selectd.name as DBUsername
, s.name as Loginname
from sys.database_principals d
inner join sys.server_principals s
on d.sid = s.sid
where s.name = 'amtpl'
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"Ya can't make an omelette without breaking just a few eggs" 😉