What it is
An essential amino acid in all protein — it only shows up on a label, with a warning, because the product contains aspartame.
Why your biology objects
Harmless and necessary for almost everyone; the "Phenylketonurics: Contains Phenylalanine" warning is for the ~1-in-10,000 born with PKU who can't metabolise it. Read it as a flag that the real molecule to weigh is the aspartame it points to, not the amino acid itself.