Welcome to the forum! Did you try opening the file in a program that can show you the text? I had this issue and fixing that worked for me:
In addition to the 2 above threads, here’s 2 others:
https://forum.inaturalist.org/t/error-code-when-trying-to-add-a-kml-file-to-create-a-new-place/14123
https://forum.inaturalist.org/t/how-to-set-up-a-5-mile-radius-5mr-project/11818/13