Let's get this sorted out, Fresh
If you are referring to the Mark as Paid button in the supplier bills that is not working, I recommend clearing the cache and cookies in your browser to address excessive data build-up in your QuickBooks Online (QBO). After that, please check if this resolves the issue.
Please see screenshot for reference:

On the other hand, if you intend to mark your invoices as paid, you must first receive the payment to update their status.
Here's how:
- Log in to your QBO account.
- Go to the Sales menu and select Invoices.

- Choose a specific customer and click Receive payment.
- Fill in the customer details such as Payment Method, Deposit, and the Payment amount that your customer has paid.

- Once done, Save and new.

I've included these resources to help you manage your customer payments and personalize your sales forms in QBO:
Should you need further help with your invoices in QBO, feel free to reach out to us again.