Why can't I add or remove Cards on a List?

If you don't have access to add or remove Cards on a List, it is likely an Auto List. 

Auto Lists have a set of requirements that automatically add Cards to the List that meet those requirements. You cannot add or remove Cards manually when it is set up as an Auto List (ie the Add Card buttons and Remove Card buttons are not available).

You can confirm it is an Auto List if there is a lightening symbol on the toolbar. 

Clicking the lightening symbol will open "Settings" where you can edit or remove your Auto List behaviours if needed.

Auto Lists are also identified with a lightening symbol on the sidebar search menu.

For more information see "What is an Auto List?" and "How do I Remove my Auto List Behaviour?".