If you see Account code *your account number* is not valid for any of your customer or supplier invoices, it means the account you have chosen to sync to is a system account.
Xero does not allow syncing to system accounts, so if you have another account number you can use, enter it in ⚙ Settings > Xero:
You may need to create a new account from one of your existing accounts, and then enter it above after clicking the Refresh Xero Data button:
