A warm welcome to the Community, haitham.
I'm here to help ensure you'll be able to change your invoice template. As an initial step, let’s check to see if the browser has a full cache. This can often cause errors and prevent web pages from loading properly.
To start with, please open QuickBooks in a private or incognito window. This prevents the browser from saving your browsing data. Here's how to open a private window in different browsers:
- 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 edit your invoice template via the private window, go back to the main browser and clear its cache to start with a clean slate.
I'm leaving you some recommended resources to guide more in managing your templates as well as your QuickBooks account:
Help Articles for QuickBooks Online.
Customize invoices, estimates, and sales receipts in QuickBooks Online
It'll be always my pleasure to help if you have any other questions. Just keep me posted in the comment section down below.