Did you download the mission from the hub and save it to your PC, then copy it to your device? Or did you log in to the hub and download it on your device?
You must be logged into the Litchi mission hub on your PC and on the app for them to sync. Ensure you are logged in on both. Then see if you can get it going.
Just a suggestion as I know you can play with Mission Hub without being logged in
How are you trying to open the mission? From the main screen, click on the FPV icon at the top left, then select "Waypoints". Then click on the small folder that appears on the left top. That opens the mission folder. Select your mission and load it.