In the Duden there's that nice verb "auserkiesen". Why is there no "Auserkiesung" and what's the official noun for that verb?
Auserkiesen (like erkiesen and kiesen) is one of the few defective verbs of the German language: Only its participle forms (auserkoren) and rarely its preterite form (erkor aus) are in current use – the present forms are unknown even to most native speakers. This is also noted by the Duden in its entry for auserkiesen:
im Infinitiv und im Präsens Aktiv ungebräuchlich
Historically this strong difference of present and past forms stems from the verb mingling with küren (source), which also provides for the verb’s default nominalisation: Kür.
Since there already is a short nice noun for this purpose and hardly anybody knows the presence stem anymore, there is no reason for another nominalisation like Auserkiesung to exist. Moreover, not even every common verb comes with a nominalisation on -ung: