Hello there, jacksalvage.
Our product engineers are currently investigating about the 400 OK error when importing miles into QuickBooks Self-Employed. They are working to identify the cause and looking for a resolution to this issue.
I'd suggest getting in touch with us again so we can add your account to the notification list. Updates are sent via email. To contact support, check out this link.
Meantime, you can add your miles manually. Let me show you how:
You can also check out these articles that will help you handle and record trips:
Thanks for your patience while we are continually working on resolving this issue. If there's anything else that you need help with you can always mention me in your reply. You take care!