DLookup causes “You cancelled the last operation” error message

The message indicates that the 3rd argument you are supplying is not correct.  It may be misspelled, lacking square brackets around problem names, lacking delimiters, lacking a value (e.g. where you concatenate a Null), lacking an operator, or something else.

This article explains how to form the 3rd argument…
    Getting a value from a table: DLookup() from Allen Browne’s site