In future, would you mind kindly trying to convert this into a reprex? (FAQ: How to do a minimal reproducible example ( reprex ) for beginners). I suspect that process will help diagnose the issue.
One issue to resolve: you are calling group_by & summarize within the parentheses of your mutate call. And calling arrange within your summarize function call. You want to move these out into separate pipe function operations.
Closer to the following,
all_trips_v3 %>%
mutate(WeekDay = ...) %>%
summarize(
Number...
Average_...
) %>%
arrange(....)