Waypoints are written directly into the unit memory (not to the SD Card) ...
Screenshots are written directly to an SD Card ...
If you are getting an error message when you hit the "Mark" button ... Then you have "Screenshots" enabled under the "Accessories" tab in the Main Menu System ...
Pressing the "Mark" button with Screenshots enabled tries to capture a screenshot at the same time as marking a waypoint ...and because the only SD Card in the unit is a mapping card ...the unit doesn't want to write a snapshot to the map card...
Disable Screenshots under the "Accessories" tab so that pressing the "Mark" button only saves a waypoint to the internal memory ...
Or install a blank SD Card in the unit that a snapshot can be written to at the same time as saving a waypoint to the unit memory ...
Rickie