Overview:
Upon end-dating a Territory in Align, the following error occurs: Cannot Change a Territory End Date to Be Less Than an Assignment's Start Date.
Root Cause:
This error means that the Territory has an assignment whose start date is after the end date the Territory is to be closed at.
Solution:
This is the expected behavior. The Territory has to be end-dated with an end date value which is equal to or after the start date of its latest assignment.
Related Documentation:
Align Help Documentation: Territory