She already hinted at this, but in addition to being flat land that's either plains, grass, or desert, there is also a minimum appeal level that the tile has to have to qualify as a suitable location for a resort, I believe it is level 4 appeal. Being next to the coast inherently adds appeal, but also being riverside, next to a mountain and next to a natural wonder. Some of your districts increase the appeal of adjacent tiles (holy sites, theater squares, and entertainment districts) so you might consider building one of these next to coastal tiles to have them qualify, but some districts (encampments, industrial zones) as well as improvements (mines, quarries) reduce the appeal. This can factor into your decision regarding whether to mill hilly forest tiles or chop and then mine them. Of course the easiest way to get tiles to qualify is to build Eiffel, which usually just makes every coastal tile qualify (that is flat and either plains/desert/grass without resource.) But aside from making them qualify for a resort, higher appeal means the resort will generate more tourism, so the districts and improvements are still a consideration even if the tile already qualifies.
As far as your tourism not working, it may just be that you haven't triggered the first point of tourism. There's a formula, someone who knows the numbers better than I do can feel free to respond, but it's something like you have to generate 150 points of tourism to get 1 "point" of foreign tourism, and you need 1 "point" of foreign tourism for every "point" of domestic tourism that they have. I'm not really fluent with the numbers yet, just know that you need a heck of a lot more tourism than your opponents are generating, which you can sidestep by wiping them out.