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 box are both checked so that the list of both the Projected and Geographic coordinate system will be enabled.
If the issue still persists, email us at email@example.com or talk with us right now from the chat bubble and we'll do our best to help.