Solve for a variable in an equation but getting an error?

Over 1 MB necessary for the question? Guess the file can condensed to 10% by deleting the pictures. We sure don't need them to deal with the problem.

P.S.: Haven't looked, but I guess you are paying the price for extensive using symbolics now.

The problem is indeed the use of the symbolics in combination with units and conditionals.

When using the symbolic solve Mathcad will trace all variables used back as far as possible up to their original definition. In case of some of your variables (like P.u_t6) Mathcad will end up at an if statement (in case of the aforementioned variables its the definition of MasonryWeight_per_ft2 in the nameless area). Mathcad's symbolic can not deal with if statements (you would rather have to use the Heaviside function instead) and so the whole solve expression fails.

You seem to have made precautions to avoid this as you have assigned

which normally breaks the connection to the original definition of P.u_t6. But because of the unit (an unknown variable to Mathcad's symbolic) the result is not seen as a number but an arithmetic expression and Mathcad will still follow back to the if-statement.

One way to break effectively the connection is to first go for a pure numeric result by dividing through the unit and add it back in a second step:

This cures the problem but you have to do it for the other variables (psi_t6 and P.u_e_t6) as well.

I knew why I suggested the numeric solve block as the better solution in the last thread 😉

P.S.: You use a lot of conditions using two if statements (if xxx="yes" do this; if xxx="no" do that). Its better to replace the second if by the otherwise statement as that way ALL possible conditions are covered.