Hello Raj ,thanks for asking this question on this platform.as from 2019 JEE will be conducted by National testing agency (NTA) twice in one year .and any student can appear for either in any phase or in both phases because the overall AIR will be made on one's best score in the exam .so if you are not going appear in first phase you can definitely appear for second phase .thanks 

Yes, You Can apply. It is not mandatory to appear in Both JEE Main January and JEE Main April. If you missed January Attempt then apply for April in the Month of February. Also, it won't affect your result in any way, If you got good marks in the second attempt then you will be eligible to get a good college. 

The advantage in appearing in both attempt is that best one among both the cores will be counted for final result calculation.