Hey there @Dr_King_Schulze! Welcome to the community! So this is likely related to a quirk in the interaction of Unreal Engine, Visual Studio, and C++ in general for large projects like this. There are some natural downstream errors that VS will detect, even if there is no problem with the actual compilation of your project. Even the official documentation recommends disregarding the entire solution error window and only focus on errors in documents within scope.
If you’d setup VS correctly, you should still be able to build your project itself without issue, though if you can’t there may be other issues at play. One common issue is installing the Unreal Engine Visual Studio Integration for recent versions that don’t support it yet like 5.4 - 5.6. Another common issue is that Live Coding can sometimes fail in some esoteric ways in which the solution can no longer be built and it needs to be cleaned and rebuilt inside of VS.
Alternatively if you have an error with the actual scripts you introduced, we can take a look here if you’d like to post them!