Nice Nice Mark！！！！！！I SOLVE IT
If y and z are variables, they should only appear on the LHS of == , <=, or >=, not =.
= is used for assignment of, or to create, an expression. == is used for equality constrains.
A symbol should be either a variable or an expression, not both,. That is why the error message was issued.
Why do you still have inv_pos in your code. If you implement the approach in section 4.3.2.1, inv_pos should not be used.
In the future, please copy and paste code using Preformatted text icon, rather than posting images (which in your case, are missing some of the lines of code).
Perhaps you should start by implementing section 4.3.2.1 method on a simpler problem. When you have that correct, thern build up to the more complicated model you want to solve.
very helpful，thanks Mark