Why, when working as root do I still need sudo in front of some commands?

[DiBosco]

I have Armbian running on an Orange Pi and I like to work by changing to su so I'm not having to constantly type sudo in front of commands when I have a lot of commands that need root privilege. However, bizzarely, sometimes I still ahve to use sudo. So for example:
 

root@orangepi5-plus:/home/Data/Mercurial_Info# usermod -a -G www-data username

bash: usermod: command not found

If I do

 

sudo usermod -a -G www-data username 

 

How can I fix this please? It's also stopping me running a script I  need that I ran on my old Banana Pi running Armbian.

[CryBaby]

This is likely a $PATH issue. usermod is in /usr/sbin which is probably not in your user's $PATH. If you su without adding - you do not get root's $PATH but with sudo you do.