The general field mapping on the host platform (e.g., Jira instance on which you have installed BigPicture) hasn't been done for a particular field.
The mapping of tasks in a particular Box is irrelevant. The check is performed just for the general mapping of the main instance.
BigPicture is installed on "nostromo2" Jira instance. The host platform mapping is:
The general mapping is on the left:
To avoid the error when sorting by "End date" or "Start date" the following fields have to be mapped:
The mapping has to be done if you want to sort tasks using the built-in field.