Try the following steps in order until the issue is resolved:
If Raiser's Edge is hosted by Blackbaud:

When hosted, this message usually indicates that the user is at or close to their maximum storage space in the Content folder of the hosting environment. Please delete any unneeded folders and attempt to merge again.

For more information about the storage limit for the hosting environment, refer to What is the data size limit for Blackbaud Hosting Services files folder?