So many times this problem happened to me when I build my Xcode generated project from Unity if I use AdMob. One of the problems is the missing _GoogleMobileAds.h file, and so many other problems. The main cause is because I never used the Cocoapod, because at first time I was thinking it was not necessary.
But the problem solved when I implement Cocoapod thingy before building the project. By running Terminal, and cd to the project directory, I run this pod command:
pod install –repo-update
After it’s done, I can build my project without any problem and successfully upload it to AppStore Connect.