It's not truly single player, but this method will enable you to play in a LAN server you host through the in-game client.

I think you can drag the mission file you download from where it's located once you DL it from Steam. (Don't ask me where it is when you subscribe, I don't even know XD I think it's in !workshop folder and the folder name is the app id for arma 3 but I would Google this part).

Then you drag the .pbo file into either the Missions folder in your appdata;
or in the Missions folder in documents.


And it should be launchable under MP > Host > LAN | 1 player slot > continue as normal
You may want to disable AI in the lobby section.

If this doesn't work lemme know maybe I can add you on Discord and help more.
I basically used a tutorial written on the steam forums to port this.

"NMD Sandbox port to another map TUTORIAL
" by DusK the DanK.

I hope this helps.

can you do a singleplayer version