Here is where Greek reveals your claim is based on a poor translation of the passage.
For your edification...
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20:23 The Great Commission not only requires supernatural power to carry it out (v. 22), but it also involves the forgiveness of sins (cf. Jer. 31:31-34, Matt. 26:28). In the similar passages in Matthew 16:19 and 18:18, the context is church discipline. Here the context is evangelism.
The second part of each conditional clause in this verse is in the passive voice and the perfect tense in the Greek text. The passive voice indicates that someone has already done the forgiving or retaining. That person must be God since He alone has the authority to do that (Matt. 9:2-3, Mark 2:7; Luke 5:21). The perfect tense indicates that the action has continuing effects; the sins stand forgiven or retained at least temporarily if not permanently.
Jesus appears to have been saying that when His disciples went to others with the message of salvation, as He had done, some people would believe and others would not. Reaction to their ministry would be the same as reaction to His had been. He viewed their forgiving and retaining the sins of their hearers as the actions of God’s agents.
If people ("any" or "anyone," plural Gr. tinon) believed the gospel, the disciples could tell the believers that God had forgiven their sins. If they disbelieved, they could tell them that God had not forgiven but retained their sins. Jesus had done this (cf. 9:39-41), and now His disciples would continue to do it. Thus their ministry would be a continuation of His ministry relative to the forgiveness of sins, as it would be in relation to the Spirit’s enablement. This, too, applies to all succeeding generations of Jesus’ disciples since Jesus was still talking about the disciples’ mission.
". . . all who proclaim the gospel are in effect forgiving or not forgiving sins, depending on whether the hearer accepts or rejects the Lord Jesus as the Sin-Bearer."
Constable, T. (2003). Tom Constable's Expository Notes on the Bible (Jn 20:23).
“The passive voice indicates that someone has already done the forgiving or retaining. “
I believe I would be remiss if I didn’t point out the following:
Indeed, the commentator is correct here, no one denies the grammar he points out. The question really still is “who does the forgiving and retaining”?
With all due respect to the commentator you cite, he engages in “begging the question” when he later asserts: “That person must be God since He alone has the authority to do that (Matt. 9:2-3, Mark 2:7; Luke 5:21)”
In other words, he engages in eisegesis here. The careful reader will note that in the passages in question, every time, it is the scribes and Pharasees who claim “only” God has the power to forgive sins. Everyone, including Catholics, knows and claims that it is *ultimately* and *through* God that all sins are forgiven. So those passages are not violated in spirit.
However to insist that the spirit of Scripture states that *only* God can forgive sin is,quite frankly, to take the side of the same scribes and Pharasees who doubted Jesus. It’s eisegetical.