Why can't the ETA be calculated for transports in the section unknown?

For transports in the "unknown" section, no ETA can be provided, mostly due to missing GPS updates (for at least 24 hours).

Additional reasons is wrong truck allocation, and allocation of licensplates in Transporeon which are not sending GPS data to Visibility Hub.

What is the difference between unassigned and upcoming?

Both statuses define future transports. The difference is that unassigned transports do not have licence plate allocated, and the upcoming transports have.