Class memoir Error: Font command \it is not supported

The Plain-TeX font-switching commands -- \rm, \sf, \tt, \bf, \it, \sc and \sl -- are not defined in the LaTeX kernel, and they are considered badly deprecated in LaTeX documents. One should use \rmfamily, \sffamily, \ttfamily, \bfseries, \itshape, \scshape, and \slshape instead.

Some, but definitely not all, LaTeX document classes do define the Plain-TeX font-switching commands. As you've noticed and report correctly, ieeeconf is such a document class. In contrast, memoir does not define these commands -- unless you provide the document class option oldfontcommands:

\documentclass[english, 11pt, a4paper, twoside, openright, oldfontcommands]{memoir}

Even then, memoir will issue warning messages whenever one of the deprecated commands is encountered.

Incidentally, neither \it nor \itshape take an argument -- they are switches. Hence, it's not entirely correct (syntactically speaking) to write

\setlist*[enumerate,1]{label=\it{\arabic*)}}

Instead, do get used to writing

\setlist*[enumerate,1]{label={\itshape\arabic*)}}

This'll work "out of the box" with all document classes, not just ieeeconf.

Use the oldfontcommands class option:

\documentclass[...,oldfontcommands]{memoir}