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:

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