Cannot add database to SQL Always ON
When named instances are having issues being connected to, there is generally an issue with one of the following:


SQL Browser isn't running
SQL Browser ports are blocked (1434 UDP)
Incorrect DNS entries
SQL TCP Port is blocked (custom port number for named instance)
SQL TCP Port is blocked (Dynamic ports)
TCP is not an enabled protocol


To workaround it, either start the SQL Browser service or unblock the port(s).

Additionally, with the SQL Server Management Studio connection dialog it is possible to press the "options" button in the lower right hand corner and override any of the preset options. In this instance, using SERVER=MyServer,Port to specify the server name and port number overrides the SQL Browser port lookup and allows for a direct connection without name to port resolution.