Check your equations.
Here's what's happening with your first constraint:
Note that the Y-axis runs from 1 to 1, which means that the expression involving those V's only produces values close to 1 over the entire range of heb values. I checked all 8 values of hph from 1 to 8 mm: All about the same.
So how will you ever get the result between 7 and 9?
I think Luc is suggesting that Mathcad can't find a solution because there isn't one (or, at least, not in the domain of interest to you). Hence, why he also suggests that you might like to check your equations and values.