You have to create a local copy of the scenario and start a new save using that local copy. If you follow the configuration guide it shows how.

Ok, i did not see this . apparently i was copying my save to steam deck when leaving to work or some outside for extended hours so i can continue playing. yet save file on steam deck offline says "(MISSING)" next to save game and throws error message. i assume it is due to my original pc linked to direct play from mod?. if so how can i connect mod save direct to offline save or vise versa?. if its not possible i tried already starting steam deck on wifi before leaving home but fully charged and not allowing it to sleep yet it starts to throw errors and hang up. I cannot keep doing this it is painful.

That I could not tell you sorry. I'm not familiar with the Steam Deck at all, nor do I believe it's supported.

But if you want to use a local copy of the scenario, you can follow the directions shown in the reforged Eden configuration guide.
You'd need to start a new save game, but as long as both your PC and steam deck have the scenario it should work then but I'm not sure.

Steam deck uses a version of Linux as it's underlying OS, with, as I understand, a custom version of Proton as it's 'translation layer' as such, EGS, is NOT supported on any Linux OS. However, your problems lie in the fact that due to the way that steam works, all workshop content rely on an active internet connection to work. As soon as you go offline, any workshop content, either scenarios or blueprints cease to function, and due to that, neither will your save files. You have to create, as Ravien has mentioned, a local copy in the scenario folder of the main game files.