How to solve Error 927 'can not open database' while configuring DB mirroring
It seems there are bugs with SSMS (2016) to configure mirroring 
https://stackoverflow.com/questions/38008208/sql-server-2016-cant-start-mirror

Try to use TSQL or SSMS 2014

https://docs.microsoft.com/en-us/sql/database-engine/database-mirroring/database-mirroring-establish-session-windows-authentication