In Ionic tried calling OneSignal.startInit, but the OneSignal plugin is not installed

I was having the same problem. Using the versions MatheusEli suggested solved it.

If this isn’t working for you, try to do cordova plugin ls & make sure it shows the correct oneSignal plugin version. Mine wasn’t, so i had to remove the old version then install the new one.

Thanks @MatheusEli , this worked for me also.