WAV files can include metadata and, in certain circumstances, this can cause an 'Invalid WAV File' message when attempting to playback via USB on a dLive or Avantis. It is possible to rerender the file to remove any metadata by following these steps:

• Load the audio file into Reaper by dragging and dropping the file into the timeline view.
• Highlight a time selection using the top bar. 
• Go to 'File > Render'…
• Use the following settings (anything not described in the text below is up to you and relates mostly to quality/speed of render):
  
  
  
  • Source: Selected tracks (stems)
  • Bounds: Time selection
  • Directory: AllenHeath-dLive\USBPLAY
    Note that rendering directly to a USB drive may be very slow, in which case, simply render to a local drive (such as your main drive) first, then copy across to this folder when rendering has completed.
  • File name: Enter a name
  • Sample rate: Supported sample rates for playback are 44.1kHz, 48kHz and 96kHz
  • Channels: Stereo
  • Format: WAV
  • WAV bit depth: Supported bit depths for playback are 16bit and 24bit
    
  • Large files: Force WAV
  • Ensure 'Write BWF ('bext') chunk' is not ticked
  • Select 'Do not include markers or regions' in the drop-down below the BWF option
  • Ensure 'Embed tempo' and all 4 other tick boxes below are not ticked
• Click 'Render file'