Not opting for an open elective course from NPTEL in AKTU does not have any direct bad impact on your results unless you are in the final semester.
If you are a final semester student it becomes necessary to enlist in NCER(non-conventional energy resources). Submit your assignments timely and pass the final exam. In case you fail to do so, your result will have a COP and you will be required to give a back paper.
I’d recommend students in all semesters enroll in elective subjects of their choice. Completion of the course will give them a certificate from NPTEL which has enough value.