Hello, Steven_ZA. 

I'll help you get this sorted out.

I've tried this on my test company and found that you'll get this error when you enter a balance when adding the stock account.

To use the account, you can enter zero in the Balance field. After saving the stock item, the balance will be automatically updated based on the initial cost of the stock item multiplied by the number of quantity on hand. 

I've got this sample screenshot for a visual reference:

I'm also adding this article that you can read for more information: Set up and track your stock in QuickBooks Online.

Please let me know if you have additional questions or other concerns in your reply. I'll be right here to answer them. Have a great day!