Is not allowable proper English

Yes, the term 'not allowable' is correct in English.

The Word 'allowable' is an adjective, a Word that describes a noun: an allowable expense, an allowable activity, etc.

The Word 'not' is an adverb that can be used to modify the adjective allowable, for example:

• The gas is an allowable expense but the sandwich is not allowable.
• It is not allowable to make personal calls while at the front desk.