Summary
CDM wizard isn't pulling in all drive mappings to the drop down list meaning that a customer can't map to the required drives.
Cause
When a customer runs through the CDM wizard they use a 365 global admin account, usually this has permission to view all the SharePoint drives in Office 365, however sometimes it does not.
Solution
Ask the customer to ensure that the global admin account has permission to view those SharePoint sites. One way you can check is to view the owner of the site from SharePoint admin in 365.