Bounce Rate is defined as Bounces upon Entries. #DIV/0!% would show up for reporting whenever Entries are 0 for a line item in a report. It is due to mathematical error as anything divided by zero is infinity. It's recommended to add Bounces and Entries as metrics to the report to validate #DIV/0!% appearing for Bounce Rate in reporting.
In case #DIV/0!% is appearing as Bounce Rate for a line item even if Entries are nonzero, Adobe Customer Care would be able to assist further.
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