# \textbackslash shows error "undefined control sequence"

I am typing the equation with \textbackslash inside as follows. It displays error "underfined control sequence". Can anyone explain me what the problem is? Thanks.

Here $x\in [0,\infty]^n$ \textblackslash  $\{1\}$. I got error "Undefined control sequence.

Here a \textbackslash b. This line is OK.