SOLVE BLOCK

To define the guess value at the top of the solve block you have to use the assignment operator (:=) not the boolean equal (=).
Furthermore you have to use the correct unit when defining the guess value, obviously it has to be a length.

Somehow the "d" in Find(d) is labelled "System". You have to manually re-label it as "Variable" to make the solve block work.

It does not make sense to use the equation v.c=v.d and provide both v.c and v.d as function arguments as the solve block must fail if the two values are not the same.. The same applies to v.a and Vd.

Furthermore, to be unit consistent, v.a would have dimension area time pressure (= Force). Guess this is not intended?

Can it be you want to look for (vary) more variables and not just d?

Maybe something like