mL is not the same as ml. The unit ml is defined, but mL is not, so in your expression
$ans11 = FormulaWithUnits("$an1 mg/mL");
the mg/mL is not recognized as a unit by FormulaWithUnits, and the expression is treated as a plain formula, in which mg and mL would be variables. But these aren't defined, so it reports that error.
If you switch to
$ans11 = FormulaWithUnits("$an1 mg/ml");
that should work.