This usually means you tried to flash an XM file onto XW hardware, or the download was corrupted. Re-download the file and check the prefix.

Ubiquiti Networks (now part of Ubiquiti Inc.) provides firmware updates for their devices through their official website. These updates often include performance improvements, new features, and security patches.

file, and wait for the process to complete without interrupting power. airMAX - Software Downloads - Ubiquiti