"I do not understand anything."
Translation:Ich verstehe nichts.
niemals = never
niemand = nobody
In your sentence, the position of the object and case of the object are wrong: keinen / niemanden (accusative).
Ich verstehe keinen / niemanden. = I do not understand anyone.
(Which, however, is a different sentence than the one you were asked to translate.)
Keiner verstehe niemals is a subjunctive form that might be used in reported speech: Er sagt, keiner verstehe niemals (He says, nobody would understand never. I.e. everbody understands sometimes....).
But even in indicative mood (Keiner versteht niemals=Nobody never understands) it's not a translation of the English sentence, see the comment of quis_lib_duo.