@Explore-Keywords As noted above you're likely going to have a hard time and need a lot of hand holding. Fortunately for you... I am recovering from hand surgery and really, really bored so I'll take a stab at it. Please be alert for typos on my end. Moving forward, please post exactly what commands you are driving and their result.
$ is the default shell "prompt" for a "mortal" user using the Bash shell. # is the default prompt for "root", a.k.a. superuser/admin. You should be attempting your upgrade logged in as your nodebb mortal user.
The stuff following the shell prompt in the ypgrade guide is what you need to be entering as commands.
Presuming you configured mongodb as directed here https://docs.nodebb.org/configuring/databases/mongo/ you are going to need to authenticate via the admin database in order to "take a dump". The command to do that will look something like this:
May well also need to specify user via the -u flag above as well, depending on how your stuff got set up. For all the nitty gritty details of the mongodump command in all their glory, drive the following command:
The ensuing verbiage is likely to be overwhelming. Do your best to sort thru it for relevant bits.
More on mongodump here, courtesy mongodb. Imagine that
Then report back.