Hi @BethMassaro ,
It seems that the module where the discussion is located has requirements.
What is interesting about your situation is that, to my knowledge, the 'mark as done' option is only available when setting up requirements for Pages and Assignments.
Keep in mind that if the module requisites were set to 'Students must complete all these requirements' + 'Students must move through requirements in sequential order', this will make you unable to mark an item as done if the previous item(s) is not completed, based on the requirement set for that item.
I hope this information helps.
Best,