I am posting a solution that I found in case anyone else is having this issue.  I spent a lot of time with Lacerte support trying to get an answer.  It is apparently a known issue (which the first agent I spoke to did not know, thus causing us to waste a lot of time): once you have one Excel file open (whether from within DMS or not), trying to open another Excel file from within DMS may result in the error shown by the previous poster.  The second agent I spoke to provided a workaround, but it did not work for me.  I noticed that the Microsoft article she sent to me which supposedly provided the workaround showed the versions of Excel it applied to, and mine (Excel 2019) was not listed.  I did some research, and the solution (at least for me), was simple:  hold down the alt key while double clicking on the Excel file that you would like to open from within DMS.  It will ask you if you would like to start a new instance of Excel, and if you click yes it will allow the Excel file to open. I hope this helps someone who is struggling with this issue so that they don't have to waste the time I did trying to find a solution.