I have encountered the same error before. At that time, the difference of GCPs' geometry coordinate and attribute field values of [Field Mappings] was the cause.
Do you import GCPs from File Geodatabase or Hosted Feature Layer? Try check on [Use Feature Geometry] under [Point feature (Class)] if it appears when you import GCPs.
I imported from CSV file. I tried importing from a feature service, it still didn't work. With a help from ESRI support, I was able to import GCPs as GCS WGS 1984 rather than State Plane. This worked for one project. I've tried the same setting with another project. It didn't work. Drone2Map was able to generate 1 m contours only. I would like to have 1 ft contours. So far, Drone2Map was able to generate 1 ft contours without importing GCPs.