Is there any reason to not put a tarp over my vegetable bed. I want to protect the bed from Black Walnut trees leave dropping into it.

Welcome to NGA, @Myrajillrussellyaho ! (whew -- I thought "weedwhacker" was a long screen name... )

As long as you don't have plants overwintering in the raised bed I see no reason why covering it with a tarp would be harmful. (If you DO have plants in the bed, I think it would be better to use something like "floating row cover" - such as Reemay, Agribond, etc -- that would let water through.)

If your raised bed is near enough for a lot of Black Walnut leaves to drop on it, you have more of a problem than just the leaves, sorry to say. The roots of the Black Walnut secrete the nasty stuff that poisons other plants - juglone. So once the roots find your raised bed, you're going to have to resort to growing things that tolerate it anyway.

So, to be honest, I don't see the point of keeping the leaves out with a tarp.

Good point, Elaine -- it's always surprising to me how far tree roots (of any kind) will travel to take advantage of my garden soil!

I made the mistake of locating a compost heap right next to a tree (pine tree?)

When I tried to turn the heap, I found that it was one big mass of fine tree roots.