Catalyzes the phosphorylation of N-acetylmannosamine (ManNAc) to ManNAc-6-P. an N-acyl-D-mannosamine + ATP = ADP + an N-acyl-D-mannosamine 6-phosphate + H(+) Amino-sugar metabolism; N-acetylneuraminate degradation; D-fructose 6-phosphate from N-acetylneuraminate: step 2/5. Homodimer. Belongs to the ROK (NagC/XylR) family. NanK subfamily.