John Gill Commentary Exodus 34:12

"Take heed to thyself, lest thou make a covenant with the inhabitants of the land whither thou goest, lest it be for a snare in the midst of thee:" — Exodus 34:12 (ASV)

Take heed to yourself
This is said not to Moses, but to the people of Israel, as a caution to them when they should enter the land of Canaan, and possess it:

lest you make a covenant with the inhabitants of the land where
you go :
enter into a league and alliance, to live friendly and amicably, and support and assist each other against the common enemy; whereas they were to smite the seven nations and destroy them, and show them no mercy, (Deuteronomy 7:1Deuteronomy 7:2)

lest it be for a snare in the midst of you ;
be the means of drawing them into the same sinful practices with themselves, especially into idolatrous ones, and so of bringing ruin and destruction on them.