Do you need to use rennet to make mozzarella curd, or will lemon juice work?
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The acid from lemon (or vinegar or citric acid) and rennet are doing subtly different things for your cheese. Both are these to help with coagulation. In fact, coagulation can happen in a lot of ways -- raw milk can coagulate just with its own natural acids. While, yes, some cheeses only need lemon, the trick is that you don't want the milk to coagulate too quickly/ at too high an acidity. Rennet allows your milk to coagulate without the acidity levels getting too high -- in other words, it can make cheese from milk that is still sweet.
For a soft cheese like chevre, queso, paneer -- lemon (or vinegar, or citric acid) can be all that you need to get the curds you want. But, for mozzarella, you need both a little initial acid (usually citric acid or you can buy starters) and a bit of rennet.
I have subbed lemon for citric acid, but I wouldn't recommend trying to use it instead of rennet -- especially for something that takes more effort/time, like mozzarella. I'd hate to do all that work and have it fail. Why were you trying to work without rennet? If you're out or can't find the kind you need, there are still great cheeses to be made that don't rely on rennet. And they do make veggie rennet if that's the issue.