You likely have the VALID_ARCHS ("Valid Architectures") build setting overridden, either for the target or for its project. This is a build setting that should get its value automatically based on which run destination you're building for. When you figure out where this override is coming from (the build setting will display in bold when overridden), select it and hit the delete key to restore it to the default value.
I was able to silence these warnings by changing my Valid Architectures setting to $(ARCHS_STANDARD) which matches my Architectures setting as well.
Oddly, this displayed as armv7 arm64 (in comparison to the default setting of armv7 armv7s arm64), but setting it to that value manually still caused the warnings. Not sure what the significance of that is, but I haven't noticed any problems in my testing yet.
Similar to other answers, I was able to remove the warning by changing $(VALID_ARCHS) (Valid Architectures) to $(ARCHS_STANDARD). From there, I cleared all Derived Data and restarted Xcode 10.0. This made the warning go away completely. Deleting everything from Valid Architectures just made a different warning appear ("No valid architectures found").
I was seeing this warning when I tried to compile code for both iOS and Mac OS.
I was able to get around it by first doing what Hassan Taleb suggested in his answer: clear out the architectures so that only valid iPhone architectures are in there. And then finally, to maintain being able to build for Mac OS, wave your mouse over the Valid Architectures so that a plus button appears. Click that and then you can add separate settings for different platforms as needed, including different CPU architectures. For macOS SDK, I can set my Valid Architectures to just x86_64 if I want.
I have seen the same warning as I building my framework.
After a while noticed these valid architecture values vary depending on the type of devices regardless the values in build settings.
Go to Target > Build Settings > Architectures >Excluded Architectures
(x84_64) .. It is used to Run a Real device And Use (arm64) to Run a Simulator For (Any SDK).
If selected Excluded Architectures > Debug and Release
To solve this problem virtually and radically, you have to choose ( Any IOS Simulator SDK) instead of (Any SDK) and set value (arm64) In this case, you can make a build and a run together without having to change the values each time.