I can see that your automation feature is turned on. This is why when you create an invoice, it automatically marked as paid when your customer has available credits.
We can open the paid invoice, and unlink the unapplied credit so it will be marked as unpaid.
Here's how to do it:
In the Sales menu from the left tab, select Invoices.
Locate and click to open the specific invoice.
Click the payment link, then select the date.
You'll be routed to the Receive Payment page. Go to the Credits section, then uncheck the unapplied payment or credit memo.
Click Save and close.
Should you want to turn off the automation feature to avoid this to happen in the future, follow these easy steps below: