You didn’t include logs, but they probably show that it failed due to lack of memory. When the rebuild fails, you can (usually) start it again with ./launcher start app.
It’s unlikely that adding that ENV would make Discourse not work. If you messed up the formatting (which is unilkely since you got discourse-setup to make the changes), it wouldn’t take a long time.
You have lots of memory free now, but the rebuild takes a lot of memory.
If you’re changing just DISCOURSE_NOTIFICATION_EMAIL you can edit it with nano and
./launcher destroy app
./launcher start app