Written between 300-500 AD, quoting earlier sources:

4. And it repented the Lord that He had made man on the earth (vi, 6). R. Judah said: [God declared:] It was a regrettable error on My part to have created him out of earthly elements, for had I created him out of heavenly elements, he would not have rebelled against Me/ R. Nehemiah interpreted it:

I am comforted (menuham) that I created him below, for had I created him above, he would have incited the celestial creatures to revolt, just as he has incited the terrestrial beings to revolt. R. Aibu interpreted: It was a regrettable error on My part to have created an evil urge (yezer ha-ra) within him, for had I not created an evil urge within him, he would not have rebelled against Me. 1 R. Levi interpreted: I am comforted that I made him from the earth.

And it grieved Him at His heart. R. Berekiah said: If a king has a palace built by an architect and when he sees it, it displeases him, against whom is he to complain? Surely against the architect! Similarly, It grieved Him at His heart.

A certain Gentile asked R. Joshua b. Karhah [mid second century]: ‘Do you not maintain that the Holy One, blessed be He, foresees the future?’ ‘Yes/ replied he. ‘But it is written, And it grieved Him at His heart?’ 4 ‘Has a son ever been born to you?’ inquired he. ‘Yes,’ was the answer. ‘And what did you do?’ — ‘I rejoiced and made all others rejoice/ he answered. ‘Yet did you not know that he would eventually die?’ ‘Gladness at the time of gladness, and mourning at the time of mourning,’ replied he. ‘Even so was it with the Holy One, blessed be He/ was his rejoinder, ‘for R, Joshua b. Levi said: Seven days the Holy One, blessed be He, mourned for His world before bringing the Flood, for it is said here, And it grieved Him, while elsewhere it says, The king grieveth for his son’ (n Sam. xix, 3). 5

1 Th.: possibly JR. Aibu translates the end of the verse thus: and it grieved Him for his (man’s) heart, i.e. the desire to evil which the heart harbours.

…

For it repenteth Me, etc. R. Abba b. Kahana observed: For it repenteth Me that I have made them and Noah — surely not! 6 Even Noah, however, was left not because he deserved it, but because
he found grace: hence, But Noah found grace in the eyes of the Lord. 7

From Book of Genesis: Chapters 1-17, Victor P. Hamilton, New International Commentary on the Old Testament

6 Viewing the debacle man has fomented, God is grieved, even to the point of experiencing pain in his heart. Note again here the echo of earlier language in Genesis. Previously Eve (3:16) and Adam (3:17) were the pain bearers. Now Yahweh himself feels that stab. Eve’s and Adam’s pain, however, is imposed due to their sin. Yahweh’s is not. Rather, his pain finds its source in the depth of the regret he experiences over fallen humanity, and in the fact that he must judge such fallenness. It is easy, of course, to dismiss such allusions as anthropopathisms, and to feel that they can tell us nothing about the essential nature of God. But verses like this remind us that the God of the OT is not beyond the capability of feeling pain, chagrin, and remorse. To call him the Impassible Absolute is but part of the truth.

Yahweh regretted [yinnāḥem] that he had made man. This point is made again in v. 7b, “I regret [ʾemḥeh] that I made him.” The AV translates nḥm as “repent.” Here we are introduced to the idea of God repenting! As a matter of fact, the Niphal of the root nḥm (as here) occurs forty-eight times in the OT, and in thirty-four of these the subject (expressed or implied) is God.3

Interestingly, the LXX usually translates Heb. nāḥam with metanoéō or metamélomai, “to be sorry, repent, change one’s mind,” but here and in v. 7 it avoids either of those verbs. It reads “And God considered that he had made man” (v. 6) and “because I have become angry that I made them” (v. 7).4 Here the LXX translators hesitated to have God repenting.

The Hebrew root in question (nḥm) is related to the noun neḥāmá, “breath” (Ps. 119:50; Job 6:10), which describes the life-giving effect of God’s word in a time of oppression. The Niphal and Hithpael stems have six basic meanings: (1) suffer emotional pain (Gen. 6:6); (2) be comforted (Gen. 37:35); (3) execute wrath (Isa. 1:24); (4) retract punishment (Jer. 18:7–8); (5) retract blessing (Jer. 18:9–10); (6) retract (a life of) sin (Jer. 8:5–6).5

It should be noted that only a few passages that speak of God’s repentance refer to God repenting over something already done. The vast majority of the instances of Yahweh’s nḥm have to do with his possible change of will concerning a future plan of action.6 This is one significant difference between God’s repentance and man’s. Still, the fact that the OT affirms that God does repent, even over a fait accompli, forces us to make room in our theology for the concepts of both the unchangeability of God and his changeability.7

Sanhedrin 108a:

The School of R. Ishmael taught: The doom [of destruction] was decreed against Noah too, but that he found favour in the eyes of God, as it is written, It repenteth me that I have made them. But Noah found grace in the eyes of the Lord.35

And the Lord was comforted that he had made man in the earth.36 When R. Dimi came37 he said: The Holy One, blessed be He, exclaimed, ‘I did well in preparing graves for them in the earth.’38 How is this signified [by the verse]? — Here is written, And the Lord was comforted;39 whilst elsewhere it is stated, And he comforted them, and spake kindly to them.40 Others say, [He exclaimed,] ‘I did not do well in establishing graves for them in the earth;’41 here it is written, And it repented the Lord; whilst elsewhere it is written, And the Lord repented of the evil which he had thought to do unto his people.42