I just starting searching a bit through the FAQ and found this line:
“Note that at the time of this writing, if you choose PWS as data source, you cannot choose ET as the weather adjustment method, because WU PWS data lacks ‘solar radiation’ parameter required by ET. Other weather adjustment methods, such as Zimmerman, can work with WU PWS.”
I was using ET so that is why it was not working. Swapped to Zimmerman and it now works.
I think the error message really needs to be improved.