I'm guessing you're talking about the Default Cube tutorial. He's using the 2.93 alpha build and you're probably using 2.92. Anytime you're not getting the same options in a feature, it's probably because of a version mismatch. I did however find a workaround in 2.92; here's what worked for me after a little experimentation:
I imagine there's a better way to do this, but it's my first time using geometry nodes. I'd love to hear constructive criticism about this setup. And, to be totally honest, I'm not sure why the Prox2 thing works. Is the Attribute Randomize node itself creating and then storing the named attribute Prox2? It has to be, right? It's not named anywhere else except the Attribute Mix node where Prox2 is added to the value of Prox. It's just a little confusing because the Math and Mix nodes both have a "Result" attribute, which clearly illustrates that you're creating/overwriting a named attribute. the Randomize node does not, so it seems like it's asking for a preexisting attribute name.
