This most likely refers Automatic Journals (Setup -> Finances) See AJ "PRINC" for reference, Formulas part.
Expressions like yours used to find Journal Type or Journal codes by Formulas per Legislation or Formulas.
Some how "PXG" formula is missing there. Take a look in SEED and/or X3 folder for reference.
Eventualy you need this "evaluate(..)" to return necessary Journal Type or Journal Code
And then try to post again!

Yes - it sounds like the 'PXG' formula is missing. I can't see any PXG formula in the default formula configuration. To check this (assuming your menu structure is similar to mine), go to Setup -> General parameters -> Formulas (it's function GESTFO). Check whether PXG exists as one of the formulas here. If not, you need to create it or as Vladimirs says, go to the PRINC automatic journal and change PXG to a formula that exists in that Setup -> General parameters -> Formulas screen.

I have experienced similar case before and got it resolved eventually after a whole lot to dig through. You can reachout for solution. The error is a system error that failed to read the exact formula in the PRINC AJ. The right formula you will find at the journal type field reads PRG but the system is seeing it as PXG. No amount of effort resolves this unless you do some form of manipulations. I had the on PU9.03. For solution, connect to me on my whatsapp line: +2348030997343 or Email: [email protected].