Only active, hourly contracts will appear in your list for time tracking purposes. You can check your contract status here. If the contract is paused, has ended, or hasn’t started yet, you can’t log time to it. If the start date is in the future, it will activate that day at UTC 00:00 (not your local time).
If the contract is hourly and active but still isn’t available in the app, it most likely recently changed status. You can force a refresh of your contract list by closing and restarting the Upwork desktop app.