It is entirely possible to mess up a ceramic coating for a variety of reasons. There are a couple of types of ceramic coating to account for, but how to apply one is something that should be thoroughly researched before going ahead and doing it.
Messing it up can come in a litany of ways. The first is if the cleaning is done incorrectly. If all of the dirt, dust, grime, etc. is not removed from the surface of the car, it will become trapped under the coating. This will make it nigh impossible to remove unless the coating is removed entirely.
The next is the issue of leaving scratches, swirl marks, and chemical imperfections. In a similar manner to the dirt, if these are not dealt with properly, they will become trapped under the surface. When you apply a coating, you want the surface to be as immaculate as you can make it.
You can also mess it up by not applying it evenly across the surface. This can create waxy-looking deposits or drip marks along the body if anywhere is too thickly applied. Or create a thin and vulnerable surface in another area if it has not been properly applied. It also needs to be left to cure for a specific amount of time. If it is not left to cure, the coating could pick up dirt and other detritus, which would then become trapped as the coating solidifies. This is also why when the coating is being left to cure, it needs to be in as pristine an environment as possible.
DIY ceramic coatings are designed to be more forgiving as they are often easier to remove, and are only intended to last a shorter period of time. As a result, they often offer a bit less protection. But doing it yourself can be a significant cost saver. However, the reason it is so expensive to be done professionally is due to the costs involved in the labor, space, and expertise.
Hopefully, this helped, and please don’t be discouraged in your journeys into detailing!