Invalid `Podfile` file: undefined method `pod' for main:Object.

如果你是在iOS中引用flutter的时候，报的这个错。建议移步 https://www.cnblogs.com/jukaiit/p/12181184.html

其他：

先 "pod setup" 再 "pod install".

还不行的话，

rm -rf ~/.cocoapods 后 "pod setup" 再 "pod install"

或者 

sudo gem install cocoapods

或者 

sudo rm -fr ~/.cocoapods/repos/master
pod setup