Common issues deploying Lead Assignment Rules via Change Sets

Possible reasons:
 

1. Assignment Rules are assigned to users which don’t exist in the target org

You may encounter the following error: 
‘Lead.Geographic Territory Assignment Rule 3 21owner or user is inactive. Org Id: 00DXXXXXXXXXXXX’

Solution:
Use Package.xml, Assign users which exist in the target to the AssignmentRules and deploy using MdAPI

2. Assignment Rules are assigned to Queues which don’t exist in the target org 

Solution:
Deploy Queues Separately via Change set or MdAPI before deploying Assignment Rules if the Assignment Rule already exists in the org

3. Assignment Rules have criteria's on Custom Object’s fields which are not included in the Change set components

Solution:
Add the non existing Custom fields in the change set components and redeploy

4. Assignment Rules have Case Teams which don’t exist in the target org

See more details on the Know Issue: 
Duplicate case team entry error on deploy of case assignment rules

5. Miscellaneous Reasons:

Rename the Assignment Rule in source org to something which doesn’t exist in the target org, upload the change set, then validate on the target org. If the data is valid and change set contains all components, validate should succeed else will throw the valid error which the customer can fix.