To ensure that the coordinate system of the Ground Control Points (GCPs) can be converted to the output coordinate system, verify the following:

1. If the GCP coordinate system is arbitrary, the output coordinate system has to be arbitrary.
2. The GCP coordinate system has to be close to the output coordinate system, i.e. they need to cover the same area on Earth.
3. If a .prj file has been used to define the GCP or output coordinate system, verify that the coordinate system is supported by the software. Compound and vertical coordinate systems are not supported.

For more information about coordinate systems: How to select / change the Image / GCP / Output Coordinate System.