I believe the error in the other topic was down to not using the correct branch of discourse (master was renamed to main). The installer should automatically swop it over, but there have been a few cases where it hasn’t.
From what I can glean from a search, you could try:
cd /var/discourse
git checkout main
And then try rebuilding?
(I’ll throw a bat signal up for @pfaffman and @Falco too as they have a lot more experience of this than I do 
)