repentanceblacklist.com says that the phrase “repent of their sins” is “never found in the Bible.”
But, if one diligently searches the Scriptures as the Bereans of Acts 17 did, one would find that the command to repent of one's sins is in the Word of God.
Jer 8:6 I hearkened and heard, but they spake not aright: no man repented him of his wickedness, saying, What have I done? every one turned to his course, as the horse rusheth into the battle.
Here, God is looking for those who would repent of their wickedness. He found none.
Eze 14:6 Therefore say unto the house of Israel, Thus saith the Lord GOD; Repent, and turn yourselves from your idols; and turn away your faces from all your abominations.
Here is a clear command to repent and turn from idols and abominations.
Ac 8:22 Repent therefore of this thy wickedness, and pray God, if perhaps the thought of thine heart may be forgiven thee.
Here, Stephen pleads with those about to stone him to repent of their wickedness.
2Co 12:21 And lest, when I come again, my God will humble me among you, and that I shall bewail many which have sinned already, and have not repented of the uncleanness and fornication and lasciviousness which they have committed.
Paul spoke of those who had not repented of the uncleanness and fornication and lasciviousness.
Re 2:21 And I gave her space to repent of her fornication; and she repented not.
Repent of fornication.
Re 2:22 Behold, I will cast her into a bed, and them that commit adultery with her into great tribulation, except they repent of their deeds.
Re 9:21 Neither repented they of their murders, nor of their sorceries, nor of their fornication, nor of their thefts.
Re 16:11 And blasphemed the God of heaven because of their pains and their sores, and repented not of their deeds.
While there is no direct quote in the Bible stating “repent of their sins,” or "repent of your sins", there are commands to repent of wickedness, uncleanness, fornication, lasciviousness, deeds, murders, sorceries, thefts, and idols.
Now, unless those things are not sins, it is clear God does want man to repent of sin.