harvmac.tex has been written in a somewhat deprecated manner, and it requires input from the console during typesetting. Texifier replaces the console with a list of errors, and one result of this is that user input is not possible during typeset.

If you do wish to use harvmac with Texifier you will need to use the following workaround:

Workaround

Firstly, Typeset with Plain TeX. As long as you are (updated to at least Texifier 1.5.0)[http://support.texifier.com/kb/installing-texpad-incl-demo-versions/texpad-os-x-distribution-in-and-out-of-the-app-store], Plain TeX is available as an option under the typeset menu.

On Texifier iOS, or Texifier macOS 1.7 and later Plain TeX is autosensed and there is no need to choose it explicitly.

Instead of choosing to typeset unreduced or reduced by typing b or l respectively on the command line, you will need to choose this in your tex file.

Typeset unreduced

To typeset in unreduced form, insert the following code before the input harvmac.tex line

\def\bigans{b }\def\read-1 to{}\def\answ{b}

Typeset unreduced

To typeset in reduced form, insert the following code before the input harvmac.tex line

\def\bigans{l }\def\read-1 to{}\def\answ{l}