There are 2 possibilities why you VAT ID is not accepted.
Company data does not match
The information that you entered in your account settings (company name and address) is different than the one stored with your tax office. You can check your VAT ID on the official website and compare the data with the one you provided in your account. You may need to update your company and address information with the tax office in case your company moved or changed the name.
Unless the company data matches, we cannot issue you an invoice without vat.
Service that checks VAT ID is temporarily down
The service that checks if your VAT ID is temporarily down. Please be patient and try updating your account data again the next day, by just clicking "Update Account". It happens from time to times that the VAT ID check service is not available in your country temporarily.
Once the VAT ID is validated the VAT is deducted from your last invoice (if not older than 31 days) and transferred back to your PayPal or credit card. Future invoices will be issued without VAT as well.