adding previous purchases to new computer

Hello,

I'm trying to add my digital folder file to digital editions and get multiple error messages, like, item is licensed to another user or content not found on this computer or  "You can only authorize this computer with an adobe id that hasn’t been previously used to authorize any other computer or mobile devices.

How do I open/use digital editions on a MacOS High Sierra 10.13.4. None of the "support" pages help with this particular issue.

thanks