Yeah! You can obviously sit for the JEE MAINS examination after qualifying the Polytechnic, but you should keep in mind that you won't be able to take admission in NIT / CFTI / IIIT, only you can be able to take the admission in IIT via JEE Advance.

Its better for you to appear for the lateral entry examination as well.

To know more about JEE MAINS visit the link below:

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

To know more about JEE ADVANCED visit the link below:

https://engineering.careers360.com/exams/jee-advanced

To know more about the Lateral Entry visit the link below:

https://engineering.careers360.com/articles/what-btech-lateral-entry-eligibility#:~:text=Tech%20lateral%20entry%20is%20the,first%20two%20semesters)%20of%20B.

I hope this answer helps. All the very best for your future endeavors!