MQL Query Error: Table Operations Join vs Outer_join

[Mehak Sharma]

Hi, 

I am looking for a solution to get an output table that displays entries that may have no value for "sent_time". It shows me the following error while using 'outer_join'. The query has no errors when I use 'join', however, that doesn't give me the desired result. 

Any suggestions/insight would be very useful. 

Thanks!