This topic is about an X18x0 mixer. Those don’t have ports to update via USB-flashdrives, so the question is off-topic.
As Wyley1 states, try to do this several times to get it into “data receiving” mode:
Hold BACK + FX CUE + TAP > Power ON
If you get this message, then start the updater again. Sometimes a re-seat of the cable triggers the next step. Also, you could try another USB port on your computer to force a successful connection.
I had this recently with my X1800 as well. It took 3 attempts…