Yeah, the error is very specific and explains exactly what it is complaining about.  IFS expects uniformity when the part is setup for sale in multiple sites.  There is a combination of site, price group and price UoM that can’t be different among sites.  So if you are trying to change the UoM on this site, it is telling you that this price group for this part is already tied to a different UoM on another site.

If you want the UoM on this site to be different, you first have to change it on the other site.  if that isn’t possible either because you are in a circular conflict, then you have to change the price group on this part on this site to something else, change the UoM on the other site, then come back to this one and change the price group and the UoM at the same time to match.

It is definitely possible to change, but you can only do it with consistency.  You may have to change them in a different order than you intended, but you can’t end up with there being different UoM for the same price group on different sites.