Names of things in code are case-sensitive. Your joint is called LeftWheel (with a capital W), but in code you wrote Leftwheel (with a lowercase W). That’s why it’s throwing that error.
When you type code into Studio, if it can identify where in the object browser you are, it will try to autocomplete names, properties, etc. It’s usually a good idea to pay attention when Autocomplete stops working, since that means Roblox doesn’t know what you’re doing anymore.
Sometimes that’s fine - code editors don’t know everything - but in this case, it should be able to fill in names and stuff once you start typing them.