Why using error trace.
Apart from the symbolic solve (where yo have the info about the result being too large to display) you have six positions with an error and every single one of them says "Variable not defined" if you click on them.
So its a good idea to look at the first one, which is the definition of D.5 with the undefined P.p
The next one is the definition of Arm.5, where you again use an undefined variable (D.e).
The next three errors are follow ups, resulting from the fact that Arm.5 and D.5 are not defined.
The last one (Embedment.fs) again uses the undefined D.e.
EDIT: Found another one. In the definition of Embedment.check you use the undefined variable M.Dr
