Timesheet Import Error: SubAccountID Cannot Be Found

​@janinakeith Check this post to get an idea. → LINK

Shall I turn this on? 

Yes ​@janinakeith  Try that option and share the results.

However, I want to know the purpose of that specific button

​@janinakeith  This check box indicates (if selected) that users can enter new values of the segmented key on data entry forms. If you clear the check box, users can specify the segmented key values on data entry forms only by typing or selecting the existing values of the segmented key and its segments.
The availability of the check box depends on the lookup mode i.e. By Segmented Key or By Segment: All Avail. Segment Values.