You solve() on four equations involving 18 variables. If it found a solution at all, it is not likely to be for the four variables you happen to want.
solve() passing in quoted strings is no longer permitted as of R2018a. In versions before that it was permitted, but that form never substitutes in numeric values from the workspace. You have have defined numeric values for 14 of the variables and that (now obsolete) form of solve() would never know. You would have needed to use subs() to import numeric values in the workspace into quoted strings.
You should rewrite your code to not use quoted strings at that point. If you are using roughly R2015a or later, you can use == where the quoted string uses = . In earlier releases, you should convert the general form 'a=b' to (a)-(b)
