Hi,
It could be the following:
The Trial Balance report has been modified. This user was granted access to this modified version through Security Settings. When he tries to print the report, GP looks to his reports dictionary to find the modified version. However the modified version is no longer in the reports dictionary (eithre it has been deleted from the dictionary, or the dictionary he is using has been changed to one that does not have the modified version in it).
To resolve: (1.) Find a reports dictionary with the modified version in it and import the modified version into the reports dictionary this user is using. or (2.) At this users PC, log into the report writer, find the Financial Trial Balance report in the left hand column and insert it into the modified list. or (3.) Remove the security setting for this user that grants access to the modified report (do this at a PC that is not having the issue - as GP will might a problem removing access to a modified report that it can't find).
Hope this helps.