Can I enter a document number on a void transaction?

No, document sequencing uses the accounting date as the document sequence date. Void transactions don't have an accounting date.

In addition, you can't void an existing transaction assigned a document number if the Receivables system option Allow transaction deletion isn't enabled. This is to maintain the integrity of gapless sequencing.

If the Receivables system option Allow transaction deletion is enabled, then voiding an existing transaction removes the document number.