Im trying to create a simple tibble of the overall mean and SD for my three variables, not per participant. Can someone help me I've been stuck on this for ages!! 
Hi,
It is rather difficult to work from a screenshot. We are more likely to be able to help with actual code and data See this FAQ
That said, it may be a bad idea to use Mean repeatedly. Try Mean1, Mean2, etc.
its a sign you've made an error that you both %>% "pipe in" a data.frame with groupings set into your dplyr::summarise call, but still also manuall name dad_items_tib with the summarise .data param. You should do one; or the other; not both !
Hope it helps.
in other words: the variables you want to use in the summarise() shouldn't be used in the group_by() .
Also, you defined "Mean" and "SD" multiple times, so you overwrite the previous results with the following ones. If your intention is to keep these seperate make them more descriptive, e.g. Mean_beh, Mean_npcr....
Still without seeing your data and lnowing what you wnat to do it's hard ot help.