Phenylketonuria is an example of an in born error of metabolism. This “error” refers to

Phenylketonuria (PKU) is a classical inborn error of metabolism caused by inherited deficiency of the enzyme phenylalanine hydroxylase (PAH). Mutations in the PAH gene reduce or eliminate enzyme activity, preventing normal conversion of the amino acid phenylalanine to tyrosine and leading to toxic accumulation of phenylalanine in blood and tissues; untreated PKU causes neurological damage. Thus, the “error” in this metabolic disorder refers to an inherited lack (or dysfunction) of a specific enzyme, not hormonal overproduction, gland atrophy, or nondisjunction chromosomal events [2].

Sources

1. [1] https://www.ncbi.nlm.nih.gov/books/NBK535378/
2. [2] https://medlineplus.gov/genetics/condition/phenylketonuria/