I found my mistake.
From my previous dspace.cfg file I copied
# consumer to maintain the search index
event.consumer.search.class = org.dspace.search.SearchConsumer
event.consumer.search.filters = Community|Collection|Item|Bundle+Add|Create|Modify|Modify_Metadata|Delete|Remove
which is replaced by the following in DSpace 6.x
# consumer to maintain the discovery index
event.consumer.discovery.class = org.dspace.discovery.IndexEventConsumer
event.consumer.discovery.filters = Community|Collection|Item|Bundle+Add|Create|Modify|Modify_Metadata|Delete|Remove
Regards
Sakib Ahmed
Assistant System Programmer
Ayesha Abed Library
Brac University
66, Mohakhali, Dhaka-1212
Phone # 88-02-9844051-4 ext.4131
Mobile # +88-015-36211341
+88-017-53758889
University Web: bracu.ac.bd