In The Trial of the Chicago 7 (2020) movie, assistant federal prosecutor Richard says:
Richard: The witness can't present them [with] testimony that would assist in making a determination of guilt or innocence.
Why shouldn't the witness present the jury with testimony which would assist in making a determination of guilt or innocence?
Alternate explanation:
Richard is saying that the witness has no "testimony that would assist in making a determination of guilt or innocence", and therefore should not be allowed to testify.
Why shouldn't witness present Jury a testimony which assist in making a determination of guilt or innocence?
They should. That's why Richard was pointing out that this one can't.
I strongly suspect that the movie dialog is a misquote at best, or is simply inaccurate. All testimony in a criminal case is supposed to help the jury (or judge) in making a determination of guilt or innocence. Testimony that does not help is irrelevant and should not even be heard. However, in general, in US courts at least, a witness is not supposed to testify to the witness's own opinion nor conclusion about the guilt or innocence of the accused. A witness is supposed to testify to facts (or opinions in the case of an expert witness) which the finder of fact may use in forming an opinion about guilt vs innocence. It may be that some statement to this effect was simplified for dramatic impact.
Edit: Given the additional context now provided in the answer by @Ross Presser, it seems thst the statement "The witness can't present them testimony that would assist in making a determination of guilt or innocence." was intended to mean that the witness could not offer any relevant testimony. Whether that claim was correct as a matter of law is, of course, a very different matter.
Adding this answer, agreeing with the others that "can't" was meant as "is unable to", but providing some more context from the subtitles. (Unfortunately I only have dialogue, not attributions.)
And, Mr. Clark, did your counterintelligence division make a report as well?
They concluded that there was no conspiracy by the defendants to incite violence during the convention.
And then what happened on the first Tuesday, after the first Monday, in November of that year?
Richard Nixon was elected president.
Sustained.
Nobody objected...
We do.
It's well-known there's no love lost between the witness and the sitting attorney general.
This witness was called to wage a political attack and should not be allowed before the jury.
Mr. Kunstler.
Your Honor, you cannot possibly be considering not allowing the jury to hear what we've just heard?
The witness can't present them testimony that would assist in making a determination of guilt or innocence.
He just testified his own Justice Department came to the conclusion...
The current Justice Department, the one that matters, came to a new conclusion. Therefore, motivation of the prosecution has to be called into question.
The motivation of the prosecution is not an issue in a courtroom.
In any courtroom I've ever been in, except this one.
