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.
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.
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.