I was informed that particular error is basically the page timing out.
Ex- You start to enter a fill-up, walk away from the keyboard and come back an hour later and Submit it. That error will display.
We're looking into the exact time frame before the page throws that error, and also for the error message itself to be a little more descriptive.
The fix, a simple page refresh. Using the browser's Back button will NOT refresh the timer. You'll need to refresh the Fill-Up page (or navigate back to it/start over).
The above error is not related to the units you are entering, and I double checked your profile and vehicle profiles to ensure the settings were set to the correct units.
Please clear your browser cache and add a fill-up again. Please let me know if you run into any other issues.