Hi,
the username and password being requested by NoMachine is your usual system account credentials which you normally use when you want to log in to your desktop locally. For example, if you normally enter your Windows desktop with your account named “Dryros” and a password, you will need to use those credentials in the NoMachine dialog box.
See Step 2 and pictures 7 or 15 here: https://www.nomachine.com/getting-started-with-nomachine
This is an essential step in order to be able to get access to the remote desktop. If you don’t know the account credentials on the computer you are accessing, you won’t be able to authenticate.