I do not know how to use Sendnotification, but I attempted to. BUT it did not work.
Source:
StarterGui > LocalScript
game:GetService("StarterGui"):SetCore("SendNotification") { Title = "Test" Text = "Test2" Duration = 500 }
Here's the fixed version...
game:GetService("StarterGui"):SetCore("SendNotification", { Title = "Test", Text = "Test2", Duration = 500 })
You were using the function incorrectly.
Moreover, the function is disabled currently.