Have you read the Dataframes documentation? You can get a column as a vector by doing df.column_name or df[:, :column_name].
Though it sounds like things won’t work while your XLSX file isn’t being read properly. From your code above it looks like things are being read in correctly, so we can’t help you further without the file itself.
Do you absolutely have to use excel? A .csv file is more robust and easier to read in.