Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Tagastē, ēs, f., a city in Numidia, south-east of Hippo Regius, the birthplace of St. Augustine, now the ruins of Tagilt, Itin. Anton. p. 44, 6.
Hence, Tagasten-sis, e, adj., of Tagaste: oppidum, Plin. 5, 4, 4, § 30: municeps, Aug. Conf. 2, 3.