I have circuit which uses: .func myf(nom,tol) (nom+nom*(gauss(tol/5)) 

and a resistor which has a value {myf(400,0.1)} 

Besides these two I have a voltage source and I can simulate this circuit on a PC. However, on MacOS it holds simulation with: Missing model definition for MYF.

 

On both computers I am using the latest version of LTspice.

Is this a known bug?

I can't verify this as I have no Mac at my disposal, but you could try using the exact specified syntax for .FUNCtions:

.func myf(nom,tol) {nom+nom*(gauss(tol/5)}

(Note the curly braces.)

It seems to make no difference on a PC,  but it may be that the Mac version has stricter syntax checking. Are you using the Mac version of LTspice, or are you using the PC version under Wine (or equivalent) on the Mac? Saying "latest version" might be confusing, as there is now a beta version of LTspice available (24.1.x), whereas the current release version is 24.0.12. It's better to be specific about what version(s) you're using on each machine.

If that doesn't reveal anything, perhaps you would consider uploading your file(s), so other people could check. We certainly have a few Mac users in this group.

--
Regards,
Tony