You've tried to add a user account, but the platform gave you an error message saying the user already exists:

"The user already exists"
This happens when the email address already exists on the WLT platform, it might have already been used by different reseller, client or external contact
Solution
This can be solved by using a plus sign before the @ sign on the email account
if you were trying to add this email address:
john@eco-deliveries-for-you.com
You can use the + sign and a label (replace uktrack with a more suitable label)
john+uktrack@eco-deliveries-for-you.com
The user will need to login with the address john+uktrack@eco-deliveries-for-you.com
however any emailed reports etc. will get delivered to john@eco-deliveries-for-you.com

This +label trick can also be used for testing purposes.
If you (the reseller user) wants to login as a client, you can follow the same process above
when you login as a reseller, i.e.
myname@mytrackingcompany.com you see the reseller view, simply navigate to the client/client user, and create a new login: