*Greetings for five days now I have been trying to log into my Linux Ubuntu? So I finally landed here. When I use this page soultion- Easily Reset Forgotten in Password Ubuntu Linux [With Video] It says Passwd user username does not exist?
HELP!
Thankyou, Boobie. 
I can’t fix your problem, but you should know that Ubu 21.04 is NOT LTS (long term release), that would be 20.04 (see chart)
So you did this :
passwd $USERNAME
where $USERNAME is the username you setup? Let’s use “fred” as an example :
passwd fred
And it returns :
passwd: user 'fred' does not exist
Sounds like you maybe didn’t mount/remount the root filesystem properly perhaps :
mount -rw -o remount /
please try the above mount operation again and NOTE very carefully any error messages.
Before typing “passwd” again - maybe verify that $USERNAME exists in the password database /etc/passwd, you can “cat /etc/passwd”, or “less /etc/passwd”, or “grep $USERNAME /etc/passwd”.
If you’re 100% sure you created that username, that it exists, then if you’re doing EVERYTHING right the way that page suggests, and your $USERNAME does NOT have an entry in /etc/passwd, then
Note : per @joseph response, I do not think there would a drastic change between 20.04 and 21.04, I can’t imagine Linux or any distribution will abandon the ancient, but reliable, /etc/passwd, /etc/shadow, /etc/group user accounting system anytime soon…
e.g.
passwd: user 'fred' does not exist
gives the same result on 20.04 and 21.04, because on NEITHER of those systems, is there a “fred” in my /etc/passwd file…
Well, I finally got to change my password. With passwd username.
And the little cutie said Password Changed. Yippie? It’s not changed or something is screwed up In Grub? And the flavor is 21.04.
Anymore suggestions will help. And BTW I’m 79 Years old. So yes I make mistakes.
I got outta bed today…
Thank You,
Bob 
I’m sure you know best…As I stated, I didn’t have an answer, I was just commenting that 21.04 isn’t LTS (Long Term Support).
