The action could not be completed because another user was creating, deleting a transaction with inventory products at exactly the same time.

I'll share information about your concern with the encountered error message in QuickBooks Online (QBO), Laraib.

As per checking, this error message you've encountered when managing transactions has been closed. With that, please ensure you're the only user when logging into your account.

After that, let's fix this by accessing your QuickBooks account in a private window to prevent the error message you've encountered from occurring. You can use either of the following keys:

• Safari: Control + Option + P
• Google Chrome: Press Ctrl + Shift + N
• Firefox or Internet Explorer: Ctrl + Shift + P

Once it goes through, you can go back to the regular browser and clear its cache to improve the method of managing your invoice transactions. If you get the same result, I suggest signing out and signing in again. You can also restart your browser or switch to another supported browser.

Additionally, you can check this article if you want to personalize your sales forms: Customize invoices, estimates, and sales receipts in QuickBooks Online.

If you have further questions about error message or any QuickBooks-related concerns. You can always reply to this thread. We're always here to help you.