How to solve an error " Invalid or missing service identifier" on launchd

When I tried to start plist file by

launchctl load -w /Library/LaunchDaemons/com.example.app.plist

it shows:
/Library/LaunchDaemons/com.example.app.plist: Invalid or missing service identifier

check the plist file.

<key>label</key>

this was the problem.

the Label must be upper capital like ;

<key>Label</key>

then run correctly.