I appreciate you for providing an error message above to help me determine the root cause of this issue, @enquiries14. 

At times, stored cache on your browser might cause unusual behavior in QuickBooks Online (QBO). That's why the system prompts an error when you run the Profit and Loss by Customer report. Rest assured that I'll share the steps with you to help fix this. 

Let's start with opening QBO using an incognito window. It doesn't store data in the cache. Thus, it's a great way to isolate browser-related issues. Depending on the browser you're using, please follow these keyboard shortcuts: 

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

Once signed in, go back to the Reports menu and run the Profit and Loss by Customer report. If it works, clear the cache of your default browser. Doing so will remove the stored cache on the browser, so it will function effectively. Otherwise, please use a different browser. 

However, if the issue persists, I'd suggest contacting Customer Care Team. They can investigate your account why you're still receiving an error when running the report. They can also help you resolve it. Just go to the Help menu at the upper right. Then, select Contact Us to connect with a live support agent.

Once fixed, you can always customise the Profit and Loss by Customer report. This lets you get the information you need about your business. Just click the Customise button at the upper right of the report screen. 

After that, you can memorise the report to save it with its current customisation settings. It helps you quickly run the report in the future. This can be done by clicking Save customisation, so it'll be saved on your Custom reports tab. The screenshot below serves as your visual guide. To learn more about this process, visit this article: Memorise Reports. 

I'm just a comment away if you need further assistance. Enjoy the rest of your day, @enquiries14.