Built-in field hasn't been mapped
The general field mapping on the host platform (e.g., the 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 primary instance.
Example
BigPicture is installed on the "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:
Built-in fields
The mapping has to be done if you want to sort tasks using the built-in field.