There are a few reasons why your lemon bars might have a crust on top. One possibility is that the sugar in the recipe didn't fully dissolve, which can happen if you use granulated sugar instead of powdered sugar. This can cause the mixture to thicken and form a crust on top. Another possibility is that the filling wasn't cooked long enough. If the filling isn't cooked long enough, it will be runny and will form a crust on top as it cools. If you're not sure what might be causing the problem, try making a test batch with just a few ingredients to see if you can identify the issue. Then, you can adjust the recipe accordingly for future batches.