The most common cause for a backup failure is the drive letter for the USB being incorrect.
Take out the USB and put it back in. You should see a popup and be shown a letter i.e. E:, F:, G: etc.
Compare this with what is in the Archive File section of the Backup Utility, if it is a different letter change it to match the USB and leave the rest, it should read like E:\R3Data.ret for a USB with E:.
Another common cause is a faulty or full USB stick.
Try using a different USB stick and do another backup. If it succeeds the other one may be full or damaged.
If you still have issues with your backup after this you should contact the support team to investigate.