Scenes in Prompto are always synced with the server so that they are available everywhere.
When a scene is set as Unavailable this means you have created a scene, perhaps saved your scene but you haven't uploaded the progress in this scene yet.
Your imported 3D model needs to be synced once so that you can open this on another computer.
To make sure that your scene is available, use the upload icon next to your save icon.
Once you have uploaded your scene, the latest version will automatically sync with the cloud and you will have the latest version of your scene everywhere.
Your scene is always available on the pc that you have created it on, even if it is nog synced yet.