onesignal library added to xcode by cocoapods.

Did you make sure to open the workspace instead of the project?
I got the same problem!
Which version of the SDK are you using? You may be using the 1.* version of the SDK and calling this method which is part of the 2.* version of the SDK. Make sure to run a pod update on the root of your project directory to get the latest version of OneSignal's iOS SDK.
I had the same problem and I had to create an instance for this class
[[OneSignal alloc] initWithLaunchOptions:launchOptions appId:@"..."];
@fernando-sendMail This is only valid for our 1.X SDK. All selectors are static on the 2.X SDK versions.
@jkasten2 I see, I followed this tutorial: https://documentation.onesignal.com/docs/ios-sdk-setup
so, instead of installing 1.13.3 I should do 2.x.x?
@fernando-sendMail 1.13.3 should only be used if you are using react native noted by it's instructions. You should leave off the version otherwise.
You should add this line:
//Add this line