I have got 266 marks in bitsat can I get mechanical in any campus or I should adopt a dual degree course

No, you can't be able to get admission in any BITS Pilani, goa and Hyderabad campus in mechanical engineering branch. your score should be above 300 for mechanical engineering branch. You can't opt dual degree courses through BITSAT only able to opt btech courses or MSc courses.
With this rank you will able to get admission in MSc courses like

• MSc physics
• MSc mathematics
• MSc chemistry

I hope this will help you