You're trying to pipe the argument in when you don't need to. The pipe, %>% passes what's on the left in as thefirst argument of the function on the right. What your code is doing is essentially this:
airquality2 <- group_by(airquality, Month)
miss_var_summary(airquality2, airquality)
What you want to do is this:
airquality %>%
group_by(Month) %>%
miss_var_summary()
(Which is the exact example from the miss_var_summary() help page)