Yes, you can appear for NEET in 2026 and 2027 as a private candidate through NIOS (National Institute of Open Schooling). The National Medical Commission (NMC) has officially confirmed that NIOS students, including private or open school candidates, are eligible for NEET UG under the Graduate Medical Education Regulations 2023. You can even take additional subjects through NIOS from authorised institutions if needed. So your plan to complete PCB through NIOS in 2026 and attempt NEET in 2027 is fully valid and allowed.