Dear panda3,
as far as I understood, BESTIndividuals is created only after the first generation is completed.
Furthermore, you use USPEX 10.4.1 and vasp6. I'm not sure whether vasp6 output format is 100% compatible with USPEX or not.
Another source of error to check is the environment LC_ALL, which may influence python3 behavior...
I hope this helps,
Carlo