As I am new to MQL4 so sometimes I will ask too basic questions, hope you dont mind. I did compile the progeam and I dont know why it display an error.
It displays an error because it seems that "not all control paths return a value" is an error now and not a warning anymore. - Just put a
statement as the last statement in start().
In general, if a function is declared and/or defined to return a value (non void function), then it has to do so.
And if you want to code in mql you basically need to know how to code in c++, since mql is a subset of c++, where all of the program flow, declarations and definitions etc are virtually the same.
Read the book: https://book.mql4.com/