Trying to write a R programming logical statement, for a specific problem

Question

I am trying to write a logical statement in R, but it is not working. I am trying to check if an argument(state) passed to function is a valid state or not. If not, I write a stop statement and throw error message.

If !(state %in% state.abb) {stop("Invalid State")}

OR

If (state !%in% state.abb) {stop("Invalid State")}

Basically, I want to use "Not in" function and using operator "!%in%" as "Not in". It is not supported.