Evaluate component :gives errors ,why?

I followed the steps of this tutorial (https://www.youtube.com/watch?v=mhYrVlvbN4k) for building a gyroid and I got this problem : the Evaluate component , gives always error and tell me that both x & y parameters have invalid name and sholud use only alphanumeric characters and not to start with a number. I try to fix this error but with no result . I am a beginner ,any suggestion ? Thanks in advance !


in the display pulldown uncheck ‘Draw Full Names’