Hi, yes, you CAN apply for Mains directly in April. It is not compulsory to give the January exam before appearing for the April one. However, since a best of the two approach is followed, I highly recommend you to give the January paper as well, just to get a brief idea of how the paper is, and this will give you a better idea about preparation required for the April attempt.

Now, if you interested in some additional knowledge and information which might help you further, the syllabus for JEE Mains 2021, it will contain most of the topics covered in class 11th and 12th in Physics, Chemistry and Mathematics. Now, one important thing to note is that  CBSE has reduced the 30% syllabus for the 2021 session due to COVID-19, so it might lead to a possible slight change in the syllabus for Mains. The exact topics will be released by the official body, but you can check the exhaustive list of topics on this link-

https://learn.careers360.com/engineering/jee-main/

The exact dates have not been released yet, thought the expected dates are expected to be similar, with JEE Mains 1 happening in January, and Mains 2 happening in April. You can check additional dates on this link if you are interested further-

https://engineering.careers360.com/articles/jee-main-exam-dates

Hope this helped!