First of all: My Images use german as a language, sorry for that but maybe you find the problem though.
So I am setting up a SQL Database on a MS SQL Server 2016 Express. While doing so I am using SQL Server Management Suite on Windows Server 2012 R2.
I established a connection to the server than clicked on "Security -> Logins -> RMB -> New Login". In the displaying picture I am chosing Windows-Authentication and add the local user I want to use.
But after I managed all settings and clicked on okay I get following error (which says that the user cannot be found which is weird because in the screen before I searched for it and was found with no problem.
I also checked the settings for the server connection (rightclick on the connection -> properties -> security -> SQL-Server and Windows-authentication)
Do you know why I it cannot find the user?
EDIT: ErrorMSG in English errormsg trying to add login via cmd