The only error message you ever get from a solve block (in versions 12 or later) is "Variable Undefined". Except by coincidence, it has nothing to do with the actual problem. If you copy and paste the expression outside the solve block, select the entire expression and hit "=" to evaluate it you will get a useful error message. Your function ro returns values with units of m, so you need to multiply x<0> by the correct length unit.
