I have caste certificatebut not cast validity certificate how can I fill my Mht cet form??

If you want to apply for MHT CET under any reserved category, you need to give them the caste validity certificate at the time of counselling otherwise you will be discarded. If you don't have one, you can easily get the certificate online. Else you have to register yourself as a general category candidate because caste validity certificate is necessary.