try to use autocomplete (ctr + space) to see the field is existing in the $Task entity or not. If not you need to add in the entity.
Did you add the CompletionDate attribute to the Task entity in 3.4.1?
If you did, check the spelling and type to make sure it matches.
Hi Emily Dedmon,
Check whether DueDate and completionDate attribute are written same way in entity as you are passing in Expression.
Also you can try to use mendix autocomplete feature by using “ctrl+space”
All the best.