Insufficient Privileges when trying to merge a case
Users receive the following error when attempting to interact with the Case Merge Premium application.
You do not have the level of access necessary to perform the operation you requested. Please contact the owner of the record or your administrator is access is necessary.
Grant Access by Profile
Navigate to Setup > Develop > Pages. Click on "Security" next to each of these pages: Duplicates, CsMrgFind, CsMrgPassThru, CsMrgSelect. Ensure the appropriate profiles are added to the "Enabled Profiles" column. Note that it is not recommended to give non-administrative profiles access to the CsMrgConfig page as that would allow someone to make changes to the configuration for Case Merge Premium.
Grant Access by User
For information on assigning permission sets, refer to Salesforce Help & Training.
Salesforce enforces security on Visualforce page access and this is expected behavior.