Is NIOS Certificate valid for Indian Army?

Yes, you can join Indian army because NIOS is having the same value as CBSE and other boards. The National Institute of Open Schooling (NIOS), formerly National Open School, (name changed in 2002) is the board of education under the Union Government of India. 

It was established by the Ministry of Human Resource Development of the Government of India in 1989 to provide education to remote areas under the motive to increase literacy and aimed forward for flexible learning. The NIOS is a national board that administers examinations for Secondary and Senior Secondary examinations of open schools similar to the CBSE and other state boards in India. Therefore NIOS Certificate is valid for Indian Army. 

Further, National Institute of Open Schooling (NIOS) has entered into an MOU (Memorandum of Understanding) with Army Educational Corps (AEC) on 14th June 2016, for a Joint Project known as NIOS Education Project for Indian Army (NEPIA) to upgrade the educational qualifications and Human Resource Quotient of Indian Army troops.