Left is a VBA function, not an Access one. However, the sequence in which
Access searches for referenced libraries essentially means that it there's a
problem with any reference, the VBA one's going to suffer (it's the last to
be checked, so that you can override functions with your own).
My standard advice is:
On the machine(s) where it's not working, open any code module (or open the
Debug Window, using Ctrl-G, provided you haven't selected the "keep debug
window on top" option). Select Tools | References from the menu bar. Examine
all of the selected references.
If any of the selected references have "MISSING:" in front of them, unselect
them, and back out of the dialog. If you really need the reference(s) you
just unselected (you can tell by doing a Compile All Modules), go back in
and reselect them.
If none have "MISSING:", select an additional reference at random, back out
of the dialog, then go back in and unselect the reference you just added. If
that doesn't solve the problem, try to unselect as many of the selected
references as you can (Access may not let you unselect them all), back out
of the dialog, then go back in and reselect the references you just
unselected. (NOTE: write down what the references are before you delete
them, because they'll be in a different order when you go back in)
Normally, I'd also mention to check that you're not using Left for any
objects of your own (fields, variable, tables, etc.), but if it used to work
and now doesn't, that's unlikely to be an issue.