Every time I try to add mileage within the app, it makes me create a vehicle, but when I try to, I get an error message and to try again later.

I've got some troubleshooting steps to get past the error message so you can add mileage in your QuickBooks Online (QBO) mobile app. 

Since you've encountered an error message when adding mileage, we'll need to make sure we update the app to the latest version. To do this, just follow the steps below (depending on your device): 

On iOS (iPhone and iPad):

1. On your device, open the App Store.
2. Click the Today tab, then select your profile.
3. In the Available updates section, look for the QuickBooks mobile app.
4. If you see the option, press Update.
5. You can also update apps manually on your Apple device.

On Android (phone or tablet): 

1. On your device, open the Google Play Store app.
2. On the home screen, tap your profile icon.
3. Click Manage apps & device. Then, hit the Manage tab.
4. Choose the Updates available filter.
5. Find the QuickBooks Online mobile app, and select the checkbox. Then tap the update icon.
6. You can also use Google's steps to update your app.

Once done, log in to your QBO mobile app and add the mileage again. You can check out this article for more details about setting up vehicles and mileage tracking. It also explains managing vehicles, tracking business miles, and adding and editing favorite locations: Set up vehicles and mileage tracking in QuickBooks Online with the mobile app.

If the error persists, we can clear your app cache in the Storage section of your mobile phone to reset the app to its default state. The piled-up cache in your application caused this odd behavior Here's how to clear app cache or data storage if you're using an Android:

1. Open your phone's Settings app.
2. Tap Apps & notifications and click on See all apps in the app Storage.
3. Select Clear storage or Clear cache. If you don't see Clear storage, tap Clear data.

For IOS:

1. Open your Settings swipe down and tap the QuickBooks Online app.
2. Swipe down again select Clear History and Website Data, and click it once again to confirm.

In case you're having problems with tracking your trips from your mobile app, just follow the steps outlined in the following article on how to resolve it: Fix mileage tracking issues in the QuickBooks Online mobile app. 

This thread's still open, so I'm all ears if you have concerns tracking your mileage or trips and vehicle info. If you also need to take care of other things like running reports, just leave a comment below. I'll be here to help you out.