what does “error : a nonstatic member reference must be relative to a specific object” mean?

EncodeAndSend is not a static function, which means it can be called on an instance of the class CPMSifDlg. You cannot write this:
CPMSifDlg::EncodeAndSend(/*…*/); //wrong – EncodeAndSend is not static

It should rather be called as:
CPMSifDlg dlg; //create instance, assuming it has default constructor!
dlg.EncodeAndSend(/*…*/); //correct

Only static functions are called with class name.
classname::Staicfunction();

Non static functions have to be called using objects.
classname obj;
obj.Somefunction();

This is exactly what your error means. Since your function is non static you have to use a object reference to invoke it.