Otherwise, switch to a different supported browser. The one you are currently on may be having a temporary issue with QuickBooks, and using a new browser for the moment will allow you to get back to work.
To know more about how the Mileage feature works in QBSE, see the below articles:
I appreciate the things that you've done to fix the issue.
It would be best to contact our Customer Care Team since you've already modified each transaction manually. This is done to check why you're not able to save the options when using the batch feature on the Miles page.
Go to Assistant and select I need help.
Enter your concern such as "Batch option in Miles page is not working" and choose I still need a human.
Follow the on-screen instructions to connect to one of our support agents.
I've added these articles for your future reference about mileage tracking troubleshooting method: