The ForeFlight Watch App requires sync to function properly. The message, "Please sign in to ForeFlight on your iPhone to continue," is displayed persistently when sync is disabled.
With the exception of Military Flight Bag (MFB) accounts, sync is enabled by default. You will only see this message if you have contacted ForeFlight support and requested to disable sync for your account. Without enabling sync, it will not be possible to use the ForeFlight Watch App.
NOTE: Sync is disabled by default for ForeFlight MFB accounts. To use the ForeFlight Watch App with an MFB account, the account owner must contact email@example.com and request to enable sync.