I have installed SQL Server 2016 Express edition and also installed SSMS 18.4, now I am having an issue while restarting the SQL Server Agent service.
I am able to login with my system user, but not able to login with the default user name like: dot (.) or (local) or sa account.
I had the same problem on a local instance of SQL Server 2017 developer edition. For me the solution was:
go to services.msc
locate SQL Server Agent service, right click properties, click Log On
tab and take note the user that is running the service**. for me it was
Local System account (the default). back out of the dialog
go to to SQL Server Management Studio, login, open the Security node,
open Login, and locate the user running the service. In my case the
user is [NT AUTHORITY\SYSTEM] which correlates to Local System above.
right click the user, properties, Server Roles, add sysadmin priv and
save.
start SQL Agent
**The user I granted sysadmin to was not the same user as the SSMS was logged in as, it was the user that was set to run the service