Try opening your QBO account on any private/incognito browser. If the same error persists, signup for a separate Melio account for free to integrate with QBO. Then try to register a bill on it.

https://affiliates.meliopayments.com/pricing

Hi AVC2,

I agree with Fiat Lux - ASIA. Let's create another bill on an incognito window. I'm here to guide you through the steps.

Sometimes local internet cache files stored in the system can cause unexpected behavior in the product. You can use these keyboard keys to open an incognito window:

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox & Microsoft Edge: Ctrl + Shift + P 
• Safari: Command + Option + P

If you're able to add a new bill, you can go back to a regular browser and clear its cache. If the same things happen, use another supported browser instead. 

I've also added this reference as your guide in managing bills: Enter and manage bills and bill payments in QuickBooks Online.

I'll be around if you have concerns about managing bills.