Login to local host / CUPS

Hi,

i would like to modify my Xerox printers setting over the CUPS interface on my new Macbook pro (1 week old), running Sierra 10.12.6.

I can open CUPS it in Safari, but when i try to save it asks me to "Log in to localhost:631". The popup demands a username and password, and i have no idea what to write in there, my normal login and pw does not work and i have not set up any other user.

Does anyone know what the problem could be?

Thank you very much!

Astrid

P.S. I tried to select my new MacBook Pro (15 inch, 2017) to my account here, but it did not show up in the list.