There are no such obligations you have to be a Btech in order to be a IAS. For UPSC CSE you need to be a graduate. You can be a graduate in any subject. So if you don't like physics and chemistry and if you have no interest in the subject then why will you waste your time. Instead you can go for a general course which will end in 3 years like BA/BSc/BCom after your 12th. That way you can learn your subject of interest as well as focus on the UPSC CSE preparations.

To know more about the UPSC CSE preparations you may visit the link below:

https://competition.careers360.com/exams/upsc-civil-services-exam

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