It might be that you have a crew member that is trying to sync their leave entered from their app with the Dashboard.

First thing to check is that it may be local issue with the device, resolved by them deleting the app and re-downloading and re-logging in.

It will most likely be that an admin has amended the following setting on the Settings : Leave page.

'Crew can request leave from the app'

Amend this to 'Yes' and you'll be golden.

Do let us know if this does not resolve.