create user account

The same way?
You just need the right account type "system administrator" for this.

how to create user accounts/ users for UAT/STG environment 

You create users in the same way as on DEV. Do you have a specific question?

i literally explained it to you. The exact same way. You just need a suitable user type for yourself. It seems that you just have a "basic user". so you cannot create it. If you don't have a "system administrator" user, you cannot create users (yes i know "add user" smartservice knowingly ignored)

One thing to keep in mind is that in Appian that each environment is indepedent, but they're all functionally equivalent. So there's nothing special about Dev / UAT/ Prod - they all work in the same way, you usually just set up different permissions in each of those environments.

So if you don't have access to create users in UAT, it likely means that your user account doesn't have the same permissions in each environment.

One thing to keep in mind is that in Appian that each environment is indepedent, but they're all functionally equivalent. So there's nothing special about Dev / UAT/ Prod - they all work in the same way, you usually just set up different permissions in each of those environments.

So if you don't have access to create users in UAT, it likely means that your user account doesn't have the same permissions in each environment.