>react-native link
warn Running `react-native link` without package name is deprecated and will be removed in next release. If you use this command to link your project assets, please let us know about your use case here: https://goo.gl/RKTeoc
info Linking "react-native-background-timer" iOS dependency
info iOS module "react-native-background-timer" has been successfully linked
info Linking "react-native-background-timer" Android dependency
info Android module "react-native-background-timer" has been successfully linked

ejectしてみる