Prism reports "Impossible weighting" when weighting would lead to division by zero for one or more point. This can happen when you choose to weight by the reciprocal of Y, and the Y value of the curve equals zero at some X values. Since division by zero is impossible, Prism reports "Impossible weighting". Go to the Methods tab and choose a different weighting method.
Prism also reports "impossible weights" when the Y value of the curve is negative for some values of X. If you weight by 1/Y, this would mean the weights would be negative. If you weight by 1/Y2, the weights would all be positive, but points with Y near zero would get lots of weight, and points with Y values far from zero (in either direction) would get low weights, and this doesn't really make sense.