Why am I unable to log in on the app, but can log in on the website?

Welcome to the Community, sally67. 

The reason why this is happening might be because of the cache in your app since you've mentioned that you can log in on your website. Please follow these steps:  

Step 1: Clear your app's cache.

For android users: 

1. Go to your device Settings. 
2. Open Apps (varies by device).
3. If you'll see the name QuickBooks app, choose it and select Storage. 
4. Click Clear cache.

For iOS: 

1. Go to Settings and click Setting General iPhone Storage.
2. Choose the QuickBooks Online App. 
3. To free up the storage without deleting the documents and data for the app, hit the Offload App.

Step 2: Uninstall and install the app. By doing these, it helps your device run optimally and can fix the problem. 

Step 3: Uninstall and reinstall the app. After doing these, enter your log in credentials that you use in your QuickBooks Online (QBO) app. 

Furthermore, you may want to learn more about solve Common Errors in the Mobile App (Android) and   Troubleshooting Quick-Start Guide for the QuickBooks Online app.

Please leave a comment if you have further concerns or information regarding QBO app. I'm here to help you.