First, ensure you have a recent backup of your USB (Main->Flash-> Flash Backup)
Another option to try is:
-Have your keyfile (.key file) handy on a different computer.
-Shutdown server, remove flash and install USB into the other computer.
-Open the USB folder and drag and drop the .key file into the /config folder.
-Then, safely eject/remove the flash and reinstall in your server and reboot.
If the issue still persists, your flash drive may be corrupted and you may need to transfer your license to a new USB. We can assist with this by contacting us at unraid.net/contact
If you start a new Unraid trial on a new flash device, please send in your new USB GUID and we can transfer the license for you.