I'm glad you made it here, r_lee1719
I want this issue to be resolved. However, I may to ask a few questions so I can provide a more accurate solution regarding your tax code issue.
I'd like to verify what country version are you using? How did you enter your tax code in QuickBooks Online? Can you show a screenshot of the exact error message? I'd appreciate any details you can provide. These information can help me identify the cause of the error.
I'm looking forward for your response. Stay safe and have a great rest of the day!