SetStage MQ106 200 worked for me just fine! - 2020 here

Doesnt work for me, "item 106 not found for parameter quest"

Works, but is fairly useless since it doesn't start the next quest

I'm having the same glitch but none of this is working. Some of the earlier stages of the quest are set to 0 when I kill Sahloknir. This might be the problem but I've tried setting them to 1 and nothing happens. The stages that are set to zero are stages 0, 1, 40, 42, 150, and 200. I need some help.

Try just reloading from before going after the dragon, then use setstage MQ106 200 and see if that completes the quest or if it just goes back to zero. You could also try reloading and doing it before even getting the quest if the stages are still causing a problem. Do you have any mods active?

I've tried 10 different new profiles with and without mods. They all have the same issue and when I use the command setstage MQ106 200, it doesn't start the next quest.

I thought you meant that the quest wasn't completing. In that case, use setstage MQ201 30 to give you the next quest ("Diplomatic Immunity") after completing "A Blade in the Dark" with the command.

I think that you might have to re-install your game if you want to complete the quest. If the quest stages are bugged on every profile then there might be something amiss that requires a re-download.