How do I correct " Something 's not right" Cannot read properties of null(reading getmonth)?

Hello there, @MEWP. Let's do some troubleshooting to sort this out.

This message usually appears when there's a runtime error. To fix this, let's troubleshoot your browser to fix this unusual behavior of the program. To begin, start by opening an incognito or private browser window. You can use the following keyboard shortcuts for the most popular browsers: 

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Microsoft Edge: Ctrl + Shift + P
• Safari: Command + Shift + N

Once you have opened the browser window, log in to your QuickBooks account and try the thing you're doing. If the message is no longer showing, proceed to clear your browser's cache. This process will remove and fix specific issues and help the program run optimally. Additionally, you can use other devices and supported browsers to ensure everything works as expected.

Moreover, can you please provide in which part of the program you encountered this message so we to provide you with an accurate resolution?

Additionally, you can check this article with the steps on personalizing your sales form: Customize invoices, estimates, and sales receipts in QuickBooks Online.