Hello userkehlee-kong!
I understand that you want to remove the Paid watermark from the invoice. Allow me to explain the reason behind it.
When you issue a $0 invoice, your Accounts Receivable is also zero. That's why QuickBooks Online will automatically mark it as paid and we're unable to remove it.
If you need help in making your sales forms more presentable, please check this link: Customise invoices, estimates, and sales receipts in QuickBooks Online.
Keep on posting here if you have more questions. We'll respond as soon as we can.