You may get this error when uploading a spreadsheet upload file:
In order for the template to recognize Dates, the format of your input in Excel needs to match the date format of your Operating System (OS). If the date formats do not match, Excel will not recognize your inputs as dates and Portfolio Manager will not be able to accept your template.
You can verify/modify the format of your Windows operating system by going to Settings > Time & language > Region > Change data formats.