It is because you don't have added pod dependency in Podfile.
you will find below line in Podfile
Uncomment this line to define a global platform for your project
#platform :ios, '9.0'
follow the below steps.
1)just remove the # .so it will be like.
platform :ios, '9.0'
2)add the below dependencies.(Do not add all ,you just need to add whatever used in you app ,if you have used Firebase/Core only then add that line only)
#Pods for app name
pod 'Firebase'
pod 'Firebase/Core'
pod 'Firebase/Auth'
pod 'Firebase/Database'
3)clean the build
flutter clean
4)run Command :
flutter run (It will automatically adds a required pods).
that's it.