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 you're stuck, you can connect with our support team by clicking the support button on the top right corner of your user portal.