Why always popup the error Error occured while starting App. Original error: Activity used to start app doesn't exist or cannot be launched! Make sure it exists and is a launchable activity

2018-08-29 14:20:26 CRITICAL Connect to Appium Server Failed:
Message: An unknown server-side error occurred while processing the command. Original error: Error occured while starting App. Original error: Activity used to start app doesn’t exist or cannot be launched! Make sure it exists and is a launchable activity

actually the activity used is correct, and it is a launchable activity, why sometimes this error popup