The easiest way to resolve this warning message is to click the Include external files checkbox under "More Options" when publishing the workbook. Doing so will wrap the Excel workbook data used into your workbook when publishing to Tableau Server. (See the very bottom of this section of the Online Help guide: http://onlinehelp.tableau.com/current/pro/desktop/en-us/help.htm#publish_workbooks_howto.html)
That said, the view will not update automatically or be able to be refreshed when new data is added to the Excel workbook. The reason for this is that the Excel workbook is located on your local computer and the Tableau Server will not necessarily have access to your computer in order to get new data from the Excel workbook. In order to enable the Tableau view to be refreshed with new data:
1. Move the Excel workbook to a shared network drive to which the Tableau Server machine has access. Your local IT team and/or Network Administrator should be able to help you determine a good location for the file.
2. Modify the data connection in the workbook to use the full file path to that file. For example: \\server.network\folder\folder\file; not the named drive like X:\file location. When asking for this path name from your IT team, the following documentation might be helpful to ensure they give you the right thing:
3. Publish the workbook to Tableau Server, with the "Include external files" option unchecked. This will maintain the live connection to the Excel file.
One other thing I should note, is that the machine where Tableau Server is installed will need the Excel drivers in order to connect to the Excel file (even after all the configuration above). Most computers have Excel installed by default, but if that machine does not, your IT team can download and install the Microsoft Excel drivers from the drivers page of the Tableau website: Driver Download | Tableau Software
Hope this helps!