I needed to reject a run because it violated a rule, but it won't let me because it says my security token is invalid. How do I make it valid?

That is probably because you either pressed reject run twice or didnt write a reason for the rejection.

Well the how do I wright a reason for my rejection.

There should be a little box that pops up when you hit reject run, if this little box isn't popping up on screen it could be due to a pop-up blocker or maybe adblocker, also if you leave the tab when you hit reject run and come back to it it will also invalidate the security token