I got an opportunity at a start up to shift from testing to development. Should I take it?

Nothing to be worried about or scared about programming. Your office is flexible and if they are offering you an opportunity you should grab it if you think it's a better opportunity than your career track. 

To grow in manual testing you will have to pick up automation anyway which involves scripting. Take the opportunity and see if it works out for you. If it does not, it's easier to switch back to testing than move to programming or developer role after gaining experience as a software testing engineer. 

So as I said, I’m a QA Intern who has good knowledge in SQL and Manual Testing. I’m learning Core Java and Selenium on weekends. Another senior employee offered me to switch from QA to Development to hopefully start working on ERP project using python based on my learning and skills. 

I’m more inclined towards QA because of my Gaines knowledge and currently also gaining knowledge. But am also interested in learning python. What should I do? Does being in both fields affect my career in a bad way? I’m a 2018 passsout and it’s been 2 months since I’m interning. I’m confused to the core! 

You can switch to python happily and if you still want to learn selenium, learn it using python as the language instead of core java.

Moving into python based / development is always good because you are at very initial stage of your career and if you switch now it helps.

I have also moved from QA to python but after 11 years of testing. It will be a bit tough initially but eventually you can grasp it.

If you are interested in development and keen to lean development , i think you should not miss this chance. 

Good Luck :)