Please check the following :

1. Check Target Account : Ensure that the same target user account is used in the planner portal and Migration Manager tool.
2. Check Exchange Online License : Check if the target tenant user account has a valid Exchange Online License.
3. Verify Manual Posting of Comments : Open target tenant's Planner Web portal using the same user account which you have used in the tool as target connector account. In the migrated plan, try to post a comment in any one of the test tasks. Close and reopen the task and check if the posted comment is displayed.
   1. If comment was posted successfully, Retry after a while : While migrating comments, the app uses Microsoft API to post comments. Due to frequent API usage, the planner system might have been restricted users from posting comments for some time (30 mins to 1 hour). So, we would recommend you to wait some time and check the case again. If you still face issues, please send us the migration job result and log file for further analysis.
   
   How to get log file
   
   How to get migration job result
   
   2. If comment was not posted, please contact Office 365 support to resolve the problem.