I encountered this issue, but the solutions provided didn't directly help me, so I'm sharing how I got myself into a similar situation and temporarily resolved it.
I created a new project within an existing solution and copy & pasted the Header and CPP file from another project within that solution that I needed to include in my new project through the IDE. Intellisense displayed an error suggesting it could not resolve the reference to the header file and compiling the code failed with the same error too.
After reading the posts here, I checked the project folder with Windows File Explorer and only the main.cpp file was found. For some reason, my copy and paste of the header file and CPP file were just a reference? (I assume) and did not physically copy the file into the new project file.
I deleted the files from the Project view within Visual Studio and I used File Explorer to copy the files that I needed to the project folder/directory. I then referenced the other solutions posted here to "include files in project" by showing all files and this resolved the problem.
It boiled down to the files not being physically in the Project folder/directory even though they were shown correctly within the IDE.
Please Note I understand duplicating code is not best practice and my situation is purely a learning/hobby project. It's probably in my best interest and anyone else who ended up in a similar situation to use the IDE/project/Solution setup correctly when reusing code from other projects - I'm still learning and I'll figure this out one day!