Thanks for the feedback! It appears you are tying to import your test cases into existing sections. Currently, the CSV import method wouldn’t support importing into multiple existing sections with the same import, as this would always create a new section as part of the import process. Section names aren’t required to be unique, and we have many teams that have duplicate section names within the same project/test suite, so the importer has been designed to always create new sections with each import if the section field/column is included.
You would only be able to import into a single section at a time with the import tool, and depending on the number of sections you have this may be the best option. You would simply leave the section column unmapped during import, and use the 'Import To" field on the first page of the import to specify the section you want to import to. This would require you to split your CSV file in to separate CSV files per section import.
Alternatively, you can import the test cases as-is with the new sections being created, and simply move the test cases using the drag&drop feature (deleting any duplicate sections afterwards). We also have plans to review adding support for importing into existing sections for a future update to TestRail.
I hope this clarifies the issue you are experiencing.