have you looked at bill.com website?  It has the steps.

Resolution for 'billcom website issue: 'How do I set-up online bill pay?' available: Yes.

The billcom website issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the billcom website issue yourself by looking for a resolution described in the article. If it is a complex billcom website issue or you are unable to solve the issue, you may contact us by clicking here or by using other billcom website support options.

For more information about resolving issues related to billcom website in Quickbooks/QBO or to get support for the issue 'How do I set-up online bill pay?', click the link below: