The Microsoft SQL Server account "REUser" has not been linked to the Raiser's Edge database
  1. Open SQL Management Studio on The Raiser's Edge SQL Server
  2. Expand Security>Logins
  3. Locate and right click on REUser
  4. Select Properties from the menu
  5. Click User Mapping from the left hand column
  6. Confirm whether this user is mapped to the current database. If the user is not mapped to the current database proceed with the next step.
  7. Expand the database for which the REUser should be added
  8. Expand Security>Users
  9. If the REUser is listed, right click and select Delete and proceed to delete the user, and schema if applicable
  10. Navigate back to Security>Logins
  11. Right click on REUser and select Properties
  12. Select User Mapping
  13. Mark the box for map next to the database in question
  14. Ensure dbo_owner and public are marked for roles
  15. Click OK
  16. Repeat for REUser_ReadOnly
The Microsoft SQL Sever instance is not configured to run using "Mixed Mode" authentication.

Change Server Authentication for the Microsoft SQL Server instance to "SQL Server and Windows Authentication mode"
(For more information on how to change your SQL server's authentication mode, please see http://technet.microsoft.com/en-us/library/ms188670.aspx)
After making this modification, you will need to restart the instance, then complete resolution option "A" above once again.