This was handled off line, but for the benefit of others:
The error is in the test column of tab 4.
The expression in that column should provide a true or false result (for example, "isblank(#myField)" or "#myField = 2"). If the expression evaluates to true, the expression in the second column is evaluated, and the result put into the mapped field in column 3.
HTH
Regards
David Evans