"This is a reversal of an accepted transaction" (Investment transactions only)
Reversal transactions are used to correct errors in transactions that you've downloaded from your brokerage.
If you attempt to delete a reversal for an erroneous transaction that you've already accepted into your transaction list, Quicken warns you before allowing you to delete the reversal. In most cases, you should accept a reversal transaction, unless you believe the reversal itself is an error. If this is the case, contact your brokerage to confirm the error.
In most cases you should click No, and then accept the reversal.