Can't complete notification mission in flutter

I’m using flutter to complete my notification mission, but it starts giving all type of Gradle errors when I’m trying to run it for test. I added Backendless flutter SDK without any problem and I could use it to work with databases flawlessly. but for notifications, when I add Backendless notification service to AppManifest.xml , it starts giving me Gradle errors that are not even related to added libraries.
How can I setup basics for notification feature in flutter?

I’d start by googling the errors you get and see if you can resolve them. Stackoverflow is a great resource for finding the necessary information.