Short answer. Google Earth Pro saves every placemark in the folder you export, and DJI Pilot only accepts one simple polygon per file. Export a single polygon — or combine them into one multi geometry — and it will import.
Export one polygon at a time
Right click the individual polygon, not the folder, and choose Save Place As → KML. Saving the folder bundles every placemark in it, which DJI rejects.
Or combine them into one geometry
If you need the areas in a single file: select the polygons, right click, choose Combine into Multi Geometry, then save that single placemark as KML.
Save as KML, not KMZ
KMZ is a zip container and may carry image overlays and styling that Pilot won't parse. Plain KML is the safer export.
Keep the shape simple
Avoid holes, inner rings and shapes drawn inside other shapes. One closed boundary.
Put it in the root of the SD card
Not in a folder. This alone accounts for a lot of the reported failures.
Faster route
Drop the Google Earth Pro file into the KML Converter — it separates the placemarks and outputs one DJI-ready file per polygon.