@minaeimi,
It seems that engine can’t recognize a date in your string, and as a result can’t apply required formatting to it. To get a desired result, you need to remove a 00:00:00 CET
substring from your data string:
Document doc = new Document(@"C:\Temp\Mina_test2.docx");
String[] fieldNames = new String[] { "LQCASE_APPDATE" };
String s = "Tue Nov 02 00:00:00 CET 2021";
Object[] fieldValues = new Object[] { s.Remove(12, 12) };
doc.MailMerge.Execute(fieldNames, fieldValues);
doc.Save(@"C:\Temp\Mina_test2-merged.docx");
If all of your data in your database share the same format as a string above, then you can use String.Remove(12, 12) method as a workaround to get a date with desired format in an output document after a mail merge.