1. error: expected ';' after expression OscCalculatorPMNS myProb; 2. error: use of undeclared identifier 'myProb' OscCalculatorPMNS myProb;

ROOT
6

The “OscCalculatorPMNS::SetMixPMNS” method is not defined in your “OscCalculatorPMNS.C” file.
Check the “OscCalculatorPMNS.h” file (all available methods should be declared there).

From your “OscCalculatorPMNS.C” file, remove:

int main(void)
{}

BTW. See “code tags” in: Posting code? Read this first!