Hi @Stephanie_Nebenzahl , welcome to the forum 
You should be able to enable this once you remove the ‘Completion status’ filter which you have enabled above. The tooltip should explain than once you hover your mouse.
Having said that, I’m not sure why this behaves likes this since I can add the ‘Completion status’ filter back on after enabling the ‘Completed on’ filter.
If you don’t mind, you could create a post in the Report a Bug section.