Power Query: Always getting DataFormat.Error We couldn't convert to Number

I'm afraid I don't follow. There is no Boolean (TRUE/FALSE) column, and if something is being converted into Boolean, then something else is going wrong earlier on. The "value column", which the aggregation function ought to be applied to is a column of (decimal) numbers. So it certainly should be able to take the average.

On a related point, if I understand your comment correctly, you are saying that I cannot average a Boolean column to find the proportion of true values? Or, more precisely, Booleans are not cast to numbers [true = 1; false = 0] when applying numerical operations, unless one explicitly changes the type?