I had this same problem and couldn't figure it out for a long time. I tried everything on this page and others and it still didn't work. But eventually, I did find a fix.
For this to work, make sure Xcode is not running. After you've closed Xcode, open Terminal and type in the command:
xattr -rc /[The File Directory of your project found in the File Inspector of your .xcodeproj file in Xcode]/
Obviously don't put the text in brackets, just replace it with what it says. Hit enter. Don't worry if nothing shows up below the command, it didn't for me. After that, you can close out of Terminal and open Xcode. Now everything should be fine.
Note: It might take a little longer to run your project, but just wait it out.
Also note: Don't downvote this answer because it doesn't work. This is one way to fix it that worked for me, but it might not work for you because you might have something else that is broken.