When trying to upload a classification file through Classification Importer, it is seen sometimes that the upload fails with an error - Failure: First column is required to be the key.
This error mostly occurs due to the incorrect format of the file that is uploaded in the Classification Importer.
Classification Importer isn't capable of handling .xls or .xlsx files. To resolve the issue, set the Save As type to Text (Tab delimited) (*.txt) instead of Excel Workbook (*.xlsx) when saving the classification file. Try uploading the file with the Text (Tab delimited) (*.txt) format and there wouldn't be any errors observed.
Other common upload issues discussed here - https://helpx.adobe.com/analytics/kb/common-saint-upload-issues.html.