Bounce Rate is defined as Bounces upon Entries. Whenever Entries are 0 for a line item in a report, it returns #DIV/0!%. It is due to the mathematical error as anything divided by zero is infinity. Add Bounces and Entries as metrics to the report to validate #DIV/0!% that appears for Bounce Rate in reporting.
If #DIV/0!% appears as Bounce Rate for a line item even if Entries are nonzero, Adobe Customer Care assists you further.