This is a user defined error message when setting up a watch
Release : 21.2
Unfortunately there are many causes of this message but for this specific use case it was due to having duplicate attributes.
We needed to configured the watch with REAL and also specify the attribute with the ATTR tag:
((REAL(ATTR(0x6c001f2).#)/REAL(ATTR(0x6c001f4).#))*100