The "Standard Youtube License" or TOS reads
You shall not copy, reproduce, distribute, transmit, broadcast,
display, sell, license, or otherwise exploit any Content for any other
purposes without the prior written consent of YouTube or the
respective licensors of the Content.
That stipulates no "free usage" and as such, you cannot use a screenshot ("Content") without permission or license from the copyright owner anywhere in your own work or on a platform such as Wikipedia that requires permissions from the copyright owner.
You are legally OK with (preferably written) approval and license from the creator for use of a screenshot on Wikipedia, because such an written agreement will satisfy Wikipedia's requirement of owner approval for the use of another creator's work on Wikipedia and gives you written permission and license to use the work on Wikipedia. Simply contact the copyright owner and ask for permission.
If you do not have the owner's permission and still use a screenshot, and are confronted with copyright infringement by the owner or Wikipedia, your use of a screenshot may fall under Fair use. The important qualifier is may. You being able to prevail with a Fair use defense - worst case, if taken to court, or at very least, Wikipedia taking down the screenshot under a DMCA request - is not guaranteed, because a judge would make that decision, based on the evidence of the usage and their determination if, in fact, a screenshot is Fair use.
Fair use (Chapter 1 - Circular 92 | U.S. Copyright Office) stipulates that usage is legally permissible when
(3) the amount and substantiality of the portion used in relation to
the copyrighted work as a whole
is not considered to be substantial enough is an infringement of copyright of the whole work. Who determines what is what is "substantial enough"? A court. The court could take into account
(4) the effect of the use upon the potential market for or value of
the copyrighted work.
in determining your usage is Fair use, or damages if the court rules against you.
You do risk copyright infringement without clear permission from the copyright owner, and you risk Wikipedia being send a DMCA takedown notice. Whether or not you are the subject of a copyright infringement lawsuit is up to the copyright owner, and if that happens, whether you prevail is up to the court.