Statistically speaking, if you have an idea for an app, and there aren't already apps for it, you have an idea that there is no way to make money with.
I'm sure there's a lot of spaces where there's room for a better app, but I can't imagine there's much space left for money-making apps that have no competition today. I believe there are such spaces, but we're probably talking species-wide blind spots at that point, and you probably aren't the first to penetrate whatever the veil is.
If trying to come up with one of these ideas anyway, that is a good way to find one.
Ask yourself, if there is one thing that I might be able to think of, that almost nobody else in the world can, what would it be? What unique combination of experiences, perspectives, talents, etc do I have that is extremely uncommon and what unique ideas can I think of because of this?
Not super practical in this case since the odds of success are still pretty low(but not 0), but looking at things this way can be useful in other scenarios as well. It's kind of similar to how I often spin negative experiences into a positive in my head, they're still experiences and experiences can always be useful, especially if they're unique
I'm sure there's a lot of spaces where there's room for a better app, but I can't imagine there's much space left for money-making apps that have no competition today. I believe there are such spaces, but we're probably talking species-wide blind spots at that point, and you probably aren't the first to penetrate whatever the veil is.