The error, Tax code is not correct, it should be a number but the code specified is 'T0' can appear on your invoices while trying to publish them. It is typically caused by an incorrect VAT code being allocated for non-VAT registered companies.
If you are seeing this error on your invoices please follow these steps to clear it, and successfully publish:
Click the Integrate button on your company page. If you do not have access to this button, you will need to ask the billing user of the account to either amend your permissions, or carry out these steps themselves.
2. On that page, ensure the correct VAT codes have been entered into these dropdown boxes.
3. Click the green 'Save' button at the bottom
4. Republish the affected invoices