This is, indeed, a reference error, but the nature of the beast is that the REAL reference you are missing doesn't have to be that one. As I recall, the FIRST FUNCTION from ANY library triggers that message regardless of whether the highlighted function is actually in the library you are missing.
Left$ and Right$ are in a VBA library.
But you want to go to any module page, open path Tools >> References, and find which reference is missing.
(It will say MISSING: so you will know which one it is.)