"Declarative" and "declaration" are derived from the verb to declare.
"Interrogative" and "interrogation" are derived from the verb to interrogate.
"Exclamatory" and "exclamation" are derived from the verb to exclaim.
"Imperative" is derived, according to Webster, from Late Latin imperātīvus. It seems that we borrowed only the past participle, not the entire verb.
There is no "imperation" because there is no verb to impere or to imperar or however that verb might have developed if we had borrowed the base verb in English. You've found a lexical gap. English does have a handful of verbs with related meanings, such as to command, to order, to request, to entreat, to instruct, and so on. If there is a single hypernym that covers all these cases, I have no idea what it might be.
Lexical gaps happen. You can have your nice and neat rows and columns, but you can't always fill in every cell in the grid.