As @slh says above, there are extra steps involved for RT3200.
- If you have not already updated to installer 1.1.3 installer, then
- Belkin RT3200/Linksys E8450 WiFi AX discussion - #6214 by el_charlie
But as for your error, you have obviously converted the device to UBI:
But then you grabbed the incorrect, non-UBI image from FS: