How do I get a certificate of entitlement or exemption please? Just received enhanced mobility PIP

Hi, my name is steve614walton, I have just received enhanced mobility for PIP, I have to renew my car tax on my car for the first time as registered disabled, I tried at the Post office as per instructions, I took all my paperwork to include blue badge, war pension documentation, log book, MOT and statement of entitlement, I was told I need a certificate of entitlement to tax my car and it should have come automatically with my letter, I informed DWP they sent me again a copy of my statement of entitlement and no certificate, My car is due for road tax 30th this month and I still do not know or have a certificate of entitlement or is the statement of intent the certificate that the post office staff member is after and did not recognise it or is there another Government Department I need to get in touch with to get this certificate of exemption please