Why cannot create credit note in sales invoice ?

First, need to remove the credit note in sales invoice.

Before remove sales invoice must remove credit note if not the credit note will not to be reuse.

sales invoice >select the invoice (yellow button edit)

Right click remove row > save 

Select the credit note >click the delete> confirm

If want to use back the credit note running number, please go to MY Biztory > sales >next running (credit note) change the  missing running number > save.

 After done created the missing running number (credit note) , please go to My Biztory >sales next running (credit note) click the yellow button ( get last ref number) > save.