I should have mentioned, normally, when I enter the application on macOS, am in the Scheduled view.
From there, I click the plus sign, top right corner. In doing so, there is no 'section' to choose your list. But, based on what you stated, I tested by clicking the list on the left first and then clicking the plus sign. That works.
Thanks
PS: Add some stuff to your Costco list for me, please 😀