It is a very generic query. I am assuming here your query is related to intra Lte handover. Handover preparation failure can be happened at any of the following nodes:
1. Source MME
2. Target MME
3.Target SGW.
4. Target eNodeB
There can be a number of failure reasons like target eNodeB doesn't have radio resource any more to server a cater one more UE. Please refer 3GPP 36.413 section 9.2.1.3, which has list of the possible causes