Hi Didi - and welcome. The problem is that mozzarella is not an easy first (or tenth) cheese to make. The Goldilocks sweet spot for stretching the cheese is in a very small range of pH. Too acidic or too alkaline and it won't stretch. And if you are using cultures then the amount of time the cheese needs to acidify is grounded in the amount of time the culture needs to both grow and transform lactose into lactic acid. If you are using lemon juice then it is a little simpler but you still need to add enough (and not too much ) juice to acidify the milk you have, and the acidity of the milk that you have, even before you add any acidity will depend on how fresh it is so there are a number of variables that "recipes" ignore but that you cannot.
That said, I would always suggest that a beginner starts with cheeses that are far more tolerant - halloumi, for example, or squeaky curd cheese, a chevre (even if that's made with cow's milk rather than goat's).