Hey Randall,
the cause may be that
Support for modern exFAT is not installed¹ on the Onefinity Controller 
1. – would become available via fuse on the older Raspberry Pi OS version found on Onefinity controller by doing ‘apt-get install fuse exfat-fuse exfat-utils’
An alternative approach would be to connect using a remote computer. This has two advantages: 1. You don’t need a flash drive, the file you want to upload just has to be somewhere on that computer and 2. on the remote computer, you can see the Camotics 3D toolpath simulation (not available on the Onefinity Controller monitor because 3D graphics support is not installed).
Welcome to the forum!