Can you look at your system settings and confirm that you allow reporting via email? Here are the items to look for and where...
Go to Settings --> System Settings --> 9. Task Completion Reporting Policy and check the following settings are ON:
9.1 - Allow reporting actual completion date via email progress reports
9.3 - Enable one-click progress reporting on all active tasks
9.4 - Allow reporting actual and remaining effort via email progress reports
9.5 - Allow reporting percent completion via email progress reports
9.9 - Progress Reporting - Should be set to Updatable report