You must have a GPS position in order to mark a waypoint (a GPS position).
Are you inside a structure, house?
Typically, especially with these lower end 7’s and such, you need to be outside with view of the sky for GPS to work.
You can press the VIEW button until you reach a GPS screen, this will show you the GPS is locked on or searching. You may need to enable the view if you have turned that view off already.
I can get a fix inside my house but it takes a while – if you are trying to create a lot of waypoints you might want to do so in google earth and humminbird pc. You can dump them to a micro sd card from humminbird pc and load onto your head unit.