Catalyzes the conversion of GlcNAc-PP-undecaprenol into ManNAc-GlcNAc-PP-undecaprenol, the first committed lipid intermediate in the de novo synthesis of teichoic acid. N-acetyl-alpha-D-glucosaminyl-di-trans,octa-cis-undecaprenyl diphosphate + UDP-N-acetyl-alpha-D-mannosamine = H(+) + N-acetyl-beta-D-mannosaminyl-(1->4)-N-acetyl-alpha-D-glucosaminyl di-trans,octa-cis-undecaprenyl diphosphate + UDP Cell wall biogenesis; poly(ribitol phosphate) teichoic acid biosynthesis. Not essential. No teichoic acid in cell walls. Belongs to the glycosyltransferase 26 family. TagA/TarA subfamily.