Bills are not available for download

Hello.

Can I check CloudTrail event history to see if there are any errors?
https://docs.aws.amazon.com/awscloudtrail/latest/userguide/view-cloudtrail-events-console.html

There may also be a problem with your browser or PC, so try downloading the invoice from another browser or PC.

Also, if there are no problems with IAM permissions, etc., as the error message suggests, there may be a problem on the AWS side, so I recommend that you open a case with AWS Support under "Account and billing" and inquire.
Inquiries under "Account and billing" can be made free of charge.
https://docs.aws.amazon.com/awssupport/latest/user/case-management.html