Mandatory Column cannot be removed

Mandatory Column cannot be removed

Hello. Hoping someone could help me. I'm attempting to create a new view under the Activities module and I'm encountering a prompt that says the field I'm trying to remove is a "mandatory column and cannot be removed." The field in question, "Activity Type," is a custom field I created under Tasks. I have tried to change the field to not required and yet I'm still getting the same message.  I have even deleted the said field and everything is the same - It still shows up.

Does anyone know what I can do about this? Thanks.

Update: This only happens when I create a new view. Activity type does not show up if I edit an existing view after I have deleted that field.