Although I installed the gem "cocoapods" and did the "pod install" command in the correct directory (platforms/ios in my case, because we are building an IONIC app), my build which is using fastlane gym fails with this message:
library not found for -lPods-Appname
The problem was that in my Fastfile I had gym configured to use the
project parameter. I changed this -> now I use the workspace parameter instead for gym and it works.
Just in case.
Just to supplement anha1979's answer:
Not only do you have to change project to workspace, don't forget to change appname.xcodeproj to appname.xcworkspace too!