As the error says there is no function P_model_main. You also only provided coded with a function called P_model. And please dont use pictures, but copy your code.
ALso it is not recommended to use global variables. If you have multiple parameters you could for example put them all in a structure called 'par', and use that as input argument to your function P_model. For example par.vv, par.q0 et cetera.
