Unable to import an Excel or .csv file into Smartsheet

Hello, I have a file which was exported from a sql server and converted to an .xlsx file. The file only has one tab and contains 12K records. I am unable to import the file into an existing workspace in Smartsheet; upon first attempt, I can select the file, then select the primary column, but then the Smartsheet 'import' window closes without importing the file. No error messages where provided. I am on the Enterprise plan and I have a license. Can anyone offer some potential reasons why I'm unable to import?

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