I hope you can figure it out, co-op sandbox is fun
That's because were joining and not hosting. When I host the game, as my joining friend don't have Sandbox+, I can enable command for me and him. Host the game, or install Sandbox + proprely on your friend game.I joined them, but my partner said nothing came up when they tried any of the commands either, so I'm not entirely sure what's going on there.
just taking a shot in the dark, since I haven't used SB+ yet, but my guess is that another mission zone is overlapping that particular Speed Rift zone. You need to use the "Restore Virtual Steelport default zones" command to clear out all of the mission zones loaded via teleporting