Excel for DDE requires two connections and Doc is quite correct in referring to the syntax needed for sending and retrieving data from a specific spreadsheet.
Unfortunately, what is also needed is an instance of Excel with a system connection so that file open / save and similar functions can be performed. It is this instance that is failing.
Within Enterprise Guide, there is a standard setting that blocks sending system commands or operating system calls to a server. As long as the system command block is in place, SAS cannot send the string to start Excel or anything else.
There are workarounds, and a call to Tech support, or a visit to the website will shed light.
Kind regards
David