I tried using x = n, but it didn’t work. I also checked the help center about piecewise functions, but didn’t find a solution
This type of question may be better suited for the calculator support at https://help.desmos.com/hc/en-us. However, it looks like you’ve got a warning on your list, n. It probably has to do with the number of periods. You have two and you need three to define a series. The initial comma is not incorrect, but is not necessary. Either [a,...b] or [a...b] should fix your issue.