John Gill Commentary Numbers 14:34

"After the number of the days in which ye spied out the land, even forty days, for every day a year, shall ye bear your iniquities, even forty years, and ye shall know my alienation." — Numbers 14:34 (ASV)

After the number of days in which you searched the land ,
[even] forty days
For so long they were searching it, (Numbers 13:25); each day for a year ;
reckoning each day for a year, forty days for forty years, as in (Ezekiel 4:6); shall you bear your iniquities, [even] forty years :

which number is given, being a round one, otherwise it was but thirty eight years and a half before they were all cut off, and their children entered the land: and you shall know my breach of promise ;
God never makes any breach of promise; his covenant he will not break, nor alter what is gone out of his lips; men break their promises, and transgress the covenant they have made with him, but he never breaks his, (Psalms 89:34); this should rather be rendered only, "you shall know my breach"; experience a breach made upon them by him, upon their persons and families by consuming them in the wilderness: the Targum of Jonathan is, ``and you shall know what you have murmured against me;'' this same word is used in the plural in (Job 33:10), and is by the Targum rendered "murmurings" or "complaints"; and so the sense is, you shall know by sad experience the evil of complaining and murmuring against me. The Vulgate Latin version is, ``you shall know my vengeance;'' and so the Septuagint, ``you shall know the fury of my anger'' which give the sense, though not a literal version of the words.