Your unresolved external symbol error is for a function like this
void send(std::string log, std::string pas, std::string ser, std::string plik)
but the function you've shown us is like this
void send(char* log, char* pas, char* ser, char* plik)
I would guess that you have written the first version in your header file, and the second version in your source file. You need to be consistent, one version or the other, not both.