Xcode 8 "The application does not have a valid signature"

Another solution is to update the version number. It works and you don't need to clean which saves time.

I had a similar problem, unfortunately, I'd have to do a full clean frequently, a very expensive (~5-6 minutes) operation for my project and rig.

It turns out this can be caused by (during a custom build phase) adding a custom 'BuildDate' field on my plist AFTER the "Copy Bundle Resources", in order to display that value for debug purposes in the app.

Not sure if the error is intentional or an Xcode 8 bug. I also found that performing any editing, even adding and deleting a character to the code base allowed me to run without doing a full clean first.

Adding that entry to a separate plist BEFORE the "Copy Bundle Resources" build phase and then drawing from that plist for BuildDate (rather than info.plist) solved the problem for me. More details in the provided source.

Source: https://forums.developer.apple.com/thread/63955 (I do not claim full credit for this fix, but the detailed fix solved the problem for me)

It looks like an Xcode 8 bug. Cleaning the project by pressing Command+Shift+K or Product -> Clean fixes it.