[comb Brush] [cut Brush] cannot be used

[jewel3962101]

I created the hair.

I want to use [comb Brush] [cut Brush] to trim my hair.

But,

 [Error: Need to specify a valid sculpt node to groom on.]

I get an error. How can I use [comb Brush] [cut Brush]? Or tell us if there are any other ways to arrange.

Thank you.