You're level 18, so you can travel. The most common way for someone like you to make money is flying overseas, buying flowers or plushies, and flying home. You can sell them for pretty good profit.
Selling losses, that someone mentioned above, is another way, but it uses your energy which means you can't train. That's not a good long term source of money, though it can be useful short term to get some working cash.
To fly for flowers and plushies, you should live in a Private Island. Rented is fine, and I recommend you look at the ones with 3700 happy. They'll do what you need at a decent price.
The PI is important because it lets you carry 10 extra items. Base is that you can carry 5 items. With a PI, you can carry 15. Add a large suitcase (a one time purchase that will cost $10M to $10.5M) will add another 4 itms, bringing you to 19 total. Faction bonuses can add more, depending on the faction. The more items you can carry per trip, the more profit you can make for the same effort.
For practical purposes, don't bother to run anything except flower and plushies. Most other items don't make any profit and may even lose money. If you get someplace and they have no flowers or plushies, look for various types of grenades as a backup. (Not as profitable as flowers and plushies, but better than nothing.) If they don't have those either, just fly home with nothing.
To avoid going someplace that has nothing in stock, bookmark this link and check it before you decide where to go.
https://doctorn.rocks/travel-hubThis isn't hard at all. Fly, buy flowers or plushies, fly home, sell them, repeat. The more you fly, the more you make. The PI is important, and will pay for itself due to the extra items, as long as you fly regularly. You'll also benefit from higher happy while doing that.