When clicking "Map Fields" for a DAT file import, an error appears and the field mapping dialog does not appear. The error message is:
"File contains header but no valid record"
1) Verify that the proper delimiters have been selected.
2) If the delimiters are correct, then the error could be caused by a mismatch between the number of fields in the field definition line, and the number of field values in the record definition lines.
E.g. if there are 25 field names in field definition line 1, but 26 field values in the subsequent lines (lines 2 and beyond), then this error will appear.
See this help article for a support utility, "Find Bad Lines", that could help find mismatched lines:
Reference the utility "Find Bad Lines".