Hi

As per rules by government candidate who has passed the the bachelors degree programme in a recognized university with an aggregate of  50%  in all subjects taken together including languages is eligible for pgcet exam.

Which means you cannot apply, having backlog affects the application form of yours and you cannot even enter the marks. So this is the reason it is not allowed

You can surely clear the backlog and then apply for pgcet.

Hope this information helps

All the best!