The acetyl group (-COCH₃) is not acidic itself. The amino acid has both an acidic -COOH group and a basic primary amine (-NH₂) group, but in the compound the primary amine becomes a secondary amine (-NH-), who basicity will depend on surrounding atoms. (If you look at a structural diagram of the compound, the only hydrogen atom it can realistically lose to have acidity is on the -COOH.)