A basic question > clang: error: no such file or directory

Hi everyone, as the title states I have a stupid quesiton, I am a newbie on OF.
The problem is that every time I try to run an example in XCode from external addons I get the same error:
clang: error: no such file or directory
I have read almost all the related topics in the forum but I can’t find an answer.
Could you please help me?
I add that I checked for the last MacOS release both in Project and Target.
I am using XCode 9.0 with Sierra.
Thanks a lot!
Gianmaria