Do use completemulti in the package declaration.

\usepackage[...,completemulti]{automultiplechoice}

I meant don't use completemulti in the package declaration.

Thanks!

I got confused because I made the wrong assumption that I had to declare it in order to write questions in 'completemult' environment.

I meant to say:

I got confused because I made the wrong assumption that I had to declare it in order to write questions in 'questionmult' environment. But now I understand completemulti option just, adds to questionmult options.