Yes, you can apply again for admission in B.tech this year under PMSSS 2021. There is no official statement by AICTE regarding the registration form you can check here the dates as per the previous year's pattern.
As you have mentioned that you did not apply last year in 2020 and instead took admission to a private college. Thus, you have not benefited yet from the pmsss scholarship so you can apply this year and be ready with your documents required for pmsss.
read a similar question :
After joining the college can I cancel my admission? and can I apply for next year?
Join the official Telegram channel to stay updated with the registration and application news.