Ah, I see you're using both the Firebase/AdMob pod and the Google-Mobile-Ads-SDK pod. It's not necessary to use both, as they both contain the SDK code needed to load and display ads. If you're using Firebase, then Firebase/AdMob is the only AdMob-related pod you need.
Because the two pods are maintained by separate teams, every once in a while they have a temporary version mismatch due to different release schedules. I'd wager that's the cause of the issue you're seeing, and that removing Google-Mobile-Ads-SDK from your Podfile will fix it.
-Andrew