I know this question has already been asked, but I can’t find any answer that works for me! I got “UnityAds/UnityAds.h file not found” in XCode, and I made sure that Unity Ads are on. I tried searching for it in the build, but it turns out the file is not in the build. Is there some sort of setting that adds this file in the build? Please help!
It must have been a bug, because when I just turned off ads then turned them back on, it worked just fine. If anyone else tries this and still gets the error, try updating Unity, and/or update Unity Ads if you downloaded it from the Package Manager. 
That because the old Advertisement package is no longer support by Unity for iOS > 14
Just open Windows->Packages Manager->Remove the “Advertisement” package
Then install the new Advertisement package following this tutorial:
https://docs.unity.com/ads/en/manual/InstallingTheiOS14SupportPackage