If I gave neet exam in 2019 with general category can I stills give neet 2020 with sc category as I didn't have the documents last year???

Hey Rohit! Yes definitely you can do that. The NEET exam is conducted every year and every year fresh registration starts. So you can fill up your cast certificate during the time of registration after which you will be treated as a reserved category candidate.

Dear Rohit, if you will be able to produce your Caste Certificate during NEET Counselling validating your Category, you can appear for NEET 2020 in that Category. So for example, if you will have the documents that will prove that you belong to SC Category  during NEET Counselling, then you can select SC Category during filling NEET application. But if you select SC as your Category and fail to produce proofs of the same, you might be held for malpractise.