John Gill Commentary Numbers 23:20

"Behold, I have received [commandment] to bless: And he hath blessed, and I cannot reverse it." — Numbers 23:20 (ASV)

Behold, I have received [commandment] to bless
The people of Israel, to pronounce a blessing upon them, to declare them a happy people:

and he has blessed, and I cannot reverse it ;
God has blessed them, has determined to bless them, has promised to bless them, has blessed them in the victories he has given them, and will complete the blessing of them, by bringing them into the land he has given them:

so the blessings which God has designed for his spiritual Israel, and bestows upon them, are irreversible; they are blessings indeed, spiritual ones, and are for ever; he blesses them with himself, as their covenant; God, their portion here and hereafter, with Christ his Son, and all things with him, with righteousness, peace, and pardon, with his Spirit and the grace thereof, with sonship, heirship, and eternal life.