why BackendlessFCMService class is unknown in the manifest?
You may get that error if Backendless SDK is not imported into your application.
Do I need to create a BackendlessFCMService class or is it in SDK?
That class is in the SDK
I installed the SDK but I get this error again.
I implement libraries to build.gradle file and add below two lines in Application Class:
Could you show what you build.gradle file looks like?