I asked a similar question and received a decent answer but it did not pursue the logic of the assertion and its apparent lack of consistency with other statements so I don't consider this to be a duplicate.
YHVH warns the Jews being led by the angel/messenger/deputy of YHVH (whom Paul calls Christ/Messiah/Anointed One) through the wilderness/desert that they musn't rebel against against him because he will not pardon them because his "name is in him":
[Exo 23:21 ESV] (21) Pay careful attention to him and obey his voice; do not rebel against him, for he will not pardon your transgression, for my name is in him.
[1Co 10:9 ESV] (9) We must not put Christ to the test, as some of them did and were destroyed by serpents,
Yet isn't YHVH a covenant name ensuring forgiveness?:
[Gen 19:16 ASV] (16) But he lingered; and the men laid hold upon his hand, and upon the hand of his wife, and upon the hand of his two daughters, Jehovah being merciful unto him: and they brought him forth, and set him without the city.
[Exo 34:6 ASV] (6) And Jehovah passed by before him, and proclaimed, Jehovah, Jehovah, a God merciful and gracious, slow to anger, and abundant in lovingkindness and truth,
[Deu 4:31 ASV] (31) for Jehovah thy God is a merciful God; he will not fail thee, neither destroy thee, nor forget the covenant of thy fathers which he sware unto them.
[2Ch 30:9 ASV] (9) For if ye turn again unto Jehovah, your brethren and your children shall find compassion before them that led them captive, and shall come again into this land: for Jehovah your God is gracious and merciful, and will not turn away his face from you, if ye return unto him.
[Psa 103:8 ASV] (8) Jehovah is merciful and gracious, Slow to anger, and abundant in lovingkindness.
[Psa 111:4 ASV] (4) He hath made his wonderful works to be remembered: Jehovah is gracious and merciful.
[Psa 116:5 ASV] (5) Gracious is Jehovah, and righteous; Yea, our God is merciful.
[Psa 130:7-8 ASV] (7) O Israel, hope in Jehovah; For with Jehovah there is lovingkindness, And with him is plenteous redemption. (8) And he will redeem Israel From all his iniquities.
[Psa 145:8 ASV] (8) Jehovah is gracious, and merciful; Slow to anger, and of great lovingkindness.
[Jer 3:12 ASV] (12) Go, and proclaim these words toward the north, and say, Return, thou backsliding Israel, saith Jehovah; I will not look in anger upon you; for I am merciful, saith Jehovah, I will not keep anger for ever.
[Joe 2:13 ASV] (13) and rend your heart, and not your garments, and turn unto Jehovah your God; for he is gracious and merciful, slow to anger, and abundant in lovingkindness, and repenteth him of the evil.
[Jon 4:2 ASV] (2) And he prayed unto Jehovah, and said, I pray thee, O Jehovah, was not this my saying, when I was yet in my country? Therefore I hasted to flee unto Tarshish; for I knew that thou art a gracious God, and merciful, slow to anger, and abundant in lovingkindness, and repentest thee of the evil.
So given YHVH's claim that he is merciful, what is the logic of the assertion that:
- the angel will not forgive their sins
- this is because YHVH's name is "in him"