Rashi on Numbers 14:16/רש"י על במדבר י״ד:ט״ז

1מבלתי יכלת וגו׳ BECAUSE THE LORD IS UNABLE etc., — “He is unable to bring them into the land on Canaan because the inhabitants of the land are strong and mighty; and besides, one king, Pharaoh, is not like thirty-one kings, who inhabit Canaan”. — This is what they will say concerning the inhabitants of this land (these kings): מבלתי יכלת because He had no power to bring them into the Land He hath slaughtered them (Midrash Tanchuma, Sh'lach 13).

2יכלת is a noun form.