Thank you for getting back to us here on the Community page, @djurcinovska1.

The steps provided by Intuit Zac should be able to help you add an existing invoice to a project. Since everything disappears when trying to modify the transaction, I encourage replicating the steps using a private window. The stored cache on the regular browser you're using may cause this unusual experience.

To save you time, you can use either of these shortcut keys:

• Google Chrome: press Ctrl + Shift + N
• Mozilla Firefox: press Ctrl + Shift + P
• Internet Explorer: press Ctrl + Shift + P
• Safari: press Command + Shift +N

If you're able to successfully add an existing invoice to a project, go back to your regular browser and clear its cache to start fresh.

Also, using other supported browsers can help us narrow down the cause of this unusual behavior.

For additional reference about the project feature, check out this article: Set up and use the Projects feature.

Let me know if you have any other questions. I'm always here to help. Have a good day!