After looking in the internet for a solution for this error I found these solutions:
- I make sure that the target is checked
- I make sure that the file is in compile sources
I made it but still have the same error:
"_OBJC_CLASS_$_NgnEngine", referenced from:
objc-class-ref in ViewController.o
ld: symbol(s) not found for architecture i386
clang: error: linker command failed with exit code 1 (use -v to see invocation)