Here are two basic steps on how to proceed when the coordinate system input in the textbox does not seem to work:
1. Identify the EPSG Code of your ground control points. You can check it through this site: https://epsg.io/
2. Make sure the Projected and Geographic tick-boxes are both checked so that the list of both Projected and Geographic coordinate systems will be enabled.
If the issue still persists, email us at firstname.lastname@example.org or talk with us right now from the chat bubble, and we'll do our best to help.