John Gill Commentary Numbers 14:18

"Jehovah is slow to anger, and abundant in lovingkindness, forgiving iniquity and transgression; and that will by no means clear [the guilty], visiting the iniquity of the fathers upon the children, upon the third and upon the fourth generation." — Numbers 14:18 (ASV)

The Lord [is] longsuffering
Towards all men, and especially towards his own people:

and of great mercy ,
being abundant in goodness, and keeping mercy for thousands:

forgiving iniquity and transgression ,
all sorts of sin:

and by no means clearing [the guilty], visiting the iniquity of the
fathers upon the children unto the third and fourth [generation] ;
which may seem to make against the plea of Moses for mercy and forgiveness; but the reason of these words being expressed seems to be, because they go along with the others in the passage referred to, and are no contradiction to the forgiving mercy of God in a way of justice.

nor did Moses request to have the guilty cleared from punishment altogether, but that God would show mercy, at least to such a degree as not to cut off the whole nation, and leave no posterity to inherit the land; which is supposed in visiting the sin of the fathers to the third or fourth generation.