Dear Moainatour
validating Simplified Invoice on web-based validator will always show signature error as web validator validates using a certificate different from the one used for signing Simplified Invoice. Therefore, validating Simplified Invoice using web-based validator will give a certificate validation error which is an expected behavior. As you are aware, Simplified Invoices are not required to be shared in XML format with customer, therefore, customers are not expected to validate Simplified Invoices using web-based validator. Developers have SDK, Sandbox and Simulation Portal to validate Simplified Invoices. You can ignore certificate errors on Simplified Invoice while validating them on web-based validator.