Hello, @SC78. 

We recently had an issue with QuickBooks Online (QBO) customers who are unable to view and pay their invoices. 

However, this was already resolved. If some of your customers are still having this error, I suggest reaching out to our Customer Support team.

They have the tools to further check your account in a safe and secure environment why they're still having this error. 

You can follow the steps provided by colleagues on how to contact the QuickBooks Online Customer Support team. 

Let us know if there's anything that we can assist. The Community is always glad to help in any way we can. Have a wonderful day!