Hello @DavidA161 ,
Contact files will receive this error during upload when there are multiple sheets and the first one is completely blank. The contact import system will only be able to read and import from the first sheet of an XLS or CSV file. You'll need to remove these extra sheets so only the sheet you want to upload is first.
If you continue to have issues, please read through our article on properly formatting contact files prior to import, or call our general support for live assistance.
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