Hi there, @Malen.

I appreciate you considering the suggestion provided by my colleague above to fix the error. Let's perform other troubleshooting steps so you can save an invoice successfully and get you back to working order.

Before we start, please take note of the following:

• Make sure the customer isn't set up as a project.
• Make sure to select the type of progress you want to use.

To get started, try logging into your QuickBooks account using a private or a different browser. There are times that the browser is full of frequently accessed page resources, thus causing some errors and/or unusual responses. 

To use a private browser, here's how:   

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

If this works, it means that you need to clear your browser's cache so the system can start fresh.

If it still doesn't work, let's recreate the products and services items used, then try to save the invoice again. For your reference, you can watch this YouTube video for more guidance: Set Up Products and Services.

If you get the same result, I recommend contacting our Technical Support Team. This way, they can investigate this matter further and help you from there.

In case you want to learn how to send an invoice, you can read this article for your future reference: Send an Invoice.

Please know that you can always get back to this post if you have any other questions. We're always here to help.