I noticed there is a bug that when people asked about it, they were sent in the wrong direction (see final remarks below).
When trying to use SQL Server 2016 with SSMS 17.1 to create a mirrored database, the following errors might be received:
Database is not configured for database mirroring.
Database x cannot be opened. It is in the middle of a restore.
This error was a real PITA that cost me 2 weeks of searching as I am new to this DBA stuff, otherwise I would have gone the scripting path.
I think this is a bug in SSMS 17.1 because it works in SSMS 16.5.3. It took me quite some time to figure the real issue was the Management Studio itself, unfortunately. The interface of version 17.1 is much better compared to version 16.5.3!
Wrong suggestions and misleading solutions
Mostly it was the database mirroring/hadr_endpoint
TCP://servername:port setup, restoring with
NORECOVERY being forgotten in many cases, but was not the real issue when backup and restore were done correctly.