The noun form of permit is permission instead of permition. Why isn't it permition?
Nominalizations of verbs with the suffix -ion are derived from Latin, whether by direct borrowing, or through an intermediary (usually French), or by analogy.
In Latin, the -io, -ionis suffix is appended to the stem of the past participle. For instance, the past participle of portare, "carry", is portatus, stem portat-, so the derived verbs import, export, deport are nominalized as importation, exportation, deportation.
In the case of the verb mittere, "send", the past participle is missus, so the derived verbs emit, commit, permit are nominalized as emission, commission, permission.