Can't prepare lesson material. Should I cancel class?
I would like to add on top of the answers of @aparente001 and @Anonymous Mathematician.
So, first thing, in 5 hours it would be very hard to come up with something solid that you would be comfortable lecturing about; thus I second the suggestion of doing some review and exercises for now.
My opinion is: you can do it.
If you have been teaching calculus so far, chances are that you are a very competent student. You can learn almost at the same time as you will be teaching. And this does not have to be a stressful experience, I would say you can be upfront with your students and let them know that you are learning at the same time as them (they already know that you are a student too).
Actually, this can be beneficial for both parts: you) are learning a new topic and because of that you better understand the difficulties that students may come upon; they) have a TA that understands their difficulties and that is able to explain things in a way that a student would learn (it may not be the best way, but I would say that is OK).
In the case they make you questions you do not know how to answer, again, be upfront: say that it is a good question, you will think and research about it, and you will explain it later, either in the class, e-mail directly to the student, whatever you prefer.
In order for this to work, like mentioned in the other answers, you have to talk to your supervisor and let him know that you were not prepared to lecture this topic. Ask him for help about learning those concepts, or a reference to whom could provide such guidance.
Use tomorrow's class for review of what's been covered so far, including problem solving. Then go straight to your supervisor.
Do you have a graduate student employee union?
Edit (after having read the additional information in a comment -- which I just incorporated into the question):
The additional information gives me a different view of the situation than I first understood.
Now that I've gotten this fuller picture of the situation:
Prepare your discussion sections more in advance, so you'll notice in good time if a particular topic needs more careful preparation.
Try a different resource, and/or ask the professor or a more advanced student for help understanding the material.
You almost certainly aren't supposed to cancel class arbitrarily, just because you feel like it. If you admit that you are cancelling class because you weren't able to prepare adequately, you'll look bad. If you have to cancel, your safest option may be to pretend you aren't feeling well or have some other extenuating circumstances. Aside from the dishonesty, that will put a lot of pressure on you to catch up in time for the next class. (If you cancel because of illness and then have to admit you really aren't prepared to teach this material, it will look terrible.)
Instead, I'd recommend the following priorities:
If you can sort things out in the five hours before class, that's the best option. Do you have any friends who have taught the class before and could help? That would be far more efficient than trying to figure it all out yourself while in a panic.
If you can't prepare in time, can you rearrange the material a little? For example, you could insert a review of what you've done so far before you move on to game theory, or you could start by introducing just the aspects you understand, and delay the hard parts until next time.
If you are completely stuck and can't figure out any plan that won't humiliate you and waste the students' time, then it's probably best to call in sick and then work as hard as you can to catch up.