vlookup using information in Gantt chart

multiple layers of questions here.

1) vlookup is unable to locate the cell data. the data for the comparison looks the same, no odd spacing or misspelling, however, they are sub tasks in the gantt chart. I've tried both true and false for the last term in the vlookup.

2) permissions - so I'm using info from a internal only document to broadcast some dates live to a customer in a separate sheet. in the documentation for Vlookup, it makes it sound like that people in the customer doc need viewer access the internal doc inorder to see the output (if we can get it to work) is this true?