I get an error when trying to send an app to my device stating that the device is busy.
Typically this means that the device has an App Lock profile on it. Remove the App Lock profile and the app can then install.
If that doesn't work then wait for all other actions to complete. One example would be waiting for an app store update to complete. You will need to wait 10 minutes and then try again. If you continue to experience this issue, you will want to power the device off and then try again.