The extent to which something is required or mandatory [closed]
I don't think mandatoriness is necessarily wrong. It seems logical to me.
Requisiteness however might be a more elegant word.
requisiteness
NOUN
The fact of being requisite; necessity, need.
[oxforddictionaries.com]
The form I would use is "requisite."
It is mainly used in a past participle/adjectival form, but can be a noun.