I am new to inkscape, and want to make a shape thats a circle, with a slice taken out. I researched and found that the slipse tool can do that, but when I go to press the button in the top left that says switch to segment, it wont let me click it. Any ideas on how to fix?

You have to draw the circle first. Then grab one of the tiny circle handles (not a square handle) and drag. Once you drag the handle, that button becomes activated.

If you need to set the arc or segment to a specific size, you can use the spinboxes on the control bar. Or else you can just make a visual estimation as you drag the handle.

And just a hint. Once you click on the elipse tool, you can hold down your ctrl key while you drag the handle, and that will keep it as a perfect circle, otherwise the sizing of height and width can change.