You can only add a new item or edit an existing list item if the list is editable, as can be seen with the icon at the right of the list. If it is on read only, you will see an eye, if it is editable, you will see a pencil icon.

If you can edit the list, open it, click on the + button to add a new item, or select the item, click on the edit button, make the changes and then click on save.