From un- + sufficiency.

unsufficiency (uncountable)

1. Obsolete form of insufficiency
   • 1594, Richard Hooker, Of Ecclesiastical Polity, Book 2, Section 8 (at Google Books) ↗:

     Whatsoever, to make up the doctrine of man's salvation, is added as in supply of the Scripture's unsufficiency, we reject it.

   • 1876, Edward Savard, Inspector of Schools, in Statement of the public accounts of the province of Quebec and annual report of the auditor of the province, p. 219 (at Google Books) ↗:

     The pupils are wanting in regularity of attendance and there is an unsufficiency of books.

   • 1906 January–October, Joseph Conrad, chapter IX, in The Secret Agent: A Simple Tale, London: Methuen & Co., […], published 1907, →OCLC ↗; The Secret Agent: A Simple Tale (Collection of British Authors; 3995), copyright edition, Leipzig: Bernhard Tauchnitz, 1907, →OCLC ↗, page 193 ↗:

     The unsufficiency and uncandidness of his answer became painfully apparent in the dead silence of the room.