Aurender W20 clock invalid??

Are you using an external clock?  If not, there could be a menu setting for internal or external clock.  If it's set to external and you aren't using an external clock, you would get an error. 

I don't have an Aurender, but that's how my dac works. 

No external clock

you might check your menu settings. Below comes from your owner's manual.

9. Coaxial BNC Word Clock Input

Connects to the BNC output of the master/word clock. Use a 75 ohm coaxial cable. Configure word clock input preferences in the "Advanced 2" section of the Settings menu.