mam. but i had read the neet guidelines they had clearly mentioned that nios students are not allowed for neet 2020

Sarthak, we shall answer this question for you from the excerpts of the NEET 2020 Information Bulletin itself.

Page 13 of the NEET Information Bulletin says: Candidates who have passed 10+2 from Open School or as private candidates shall not be eligible to appear for ‘National Eligibility Cum Entrance Test’. Furthermore, study of Biology/Biotechnology as an Additional Subject at 10+2 level also shall not be permissible.*

There is an asterisk mark that you must have seen after the end of the sentence. This para explains that. This is also given in the Information Bulletin page no#; 13

*The candidates who have passed 10+2 level with Biology as an additional subject will also be eligible for MBBS Entrance Examination (as per Hon’ble High Court Order No. 2341/-W/DHC/WRIT/D-1/2019 dated 24/09/2019 in the Writ Petition (C) No. 6773/2019.

It further states:

*(NOTE: The proviso in italics has been subject matter of challenge before the Hon’ble High Court of Delhi, Hon’ble High Court of Allahabad, Lucknow Bench and Hon’ble High Court of Madhya Pradesh at Jabalpur. The provisions of the regulations disqualifying recognised Open School Board candidates and the candidates who have studied Biology/Biotechnology as an additional Subject has been struck down. The Medical Council of India has preferred Special Leave Petitions before the Hon’ble Supreme Court and Appeals in the Hon’ble High Courts. Therefore, the candidature of candidates in the NEET (UG)-2020 who have passed the qualifying examinations i.e. 10+2 from National Institute of Open Schooling or State Open Schools or as private candidates from recognised State Boards; or with Biology/Biotechnology as additional subject shall be allowed but subject to the outcome of Special Leave Petitions/Appeals filed by the Medical Council of India

This means that the mentioned rule is pending in the Supreme Court for its judgement and until then NIOS Candidates are eligible for appearing for NEET. What must not be missed while reading is the entire context. If NIOS Candidates are not eligible for NEET, not every website would be stating that NIOS Candidates are eligible.

In short, previously NIOS Candidates were not allowed for NEET. There was a PIL filed in the  Honourable  Court and the Court has ordered a Stay on this matter which means until the Honourable Court decides and passes on a judgement on this ruling, NIOS Candidates will be eligible to appear for NEET provided they fulfil the other eligibility criteria.