BOSSE Mark sheet validation

I have given my 12th last year in 2023 from GSEB board but could not get 75% so gave boards again 2024 from Board of open schooling and skill education,Sikkim Got 79.8% was not alloted a seat in jossa but as tomorrow is the last date for registration for csab i might get iiit or nit but I am not sure if the marksheet will be valid or not i mailed and called csab headquarters but they replied rudely on call and on mail they asked me to share the marksheet with principal’s sign but I am bot sure i could get it as its in Sikkim and I am from Ahmedabad,Gujarat Please I need help