If you have had the Frame.io Companion app while using FCPX 10.2 and then upgraded to 10.3+, your Share Destinations have changed with the FCPX 10.3+ update which means you have to renew the connection of the share destinations to the Frame.io app. Unfortunately, Frame.io cannot automate this adjustment as it's within FCPX and cannot be controlled by the Companion App.
Edit Existing Share Destinations
In Final Cut Pro X, go to File > Share. If Frame.io H.264 is still an option, select this and then you can edit the existing Share Destination.
Where it says "Open With" or "When done", choose "Other..." and then select the Frame.io app from your Applications folder.
Add New Share Destinations
If Frame.io does not currently exist on Share Destinations, go to File > Share > Add Destination and drag Export File to the Destinations on the left. The Frame.io option will disappear when you delete your Preferences.
Select the video codec and select Frame.io from Open With.