Can't Find Excel
Your simulation references an Excel file but Excel is not running, or your sheet that contains the data is not open (the sheet name should be the same as the word after the comma (,) in the message box. - if it is not either rename the sheet or change the sheet's name in the Simul8 distribution dialog that uses it).
Another reason could be that Excel is refusing to talk to Simul8. This can happen if you are in the middle of editing something in Excel (e.g. a cell value or a macro).