Do we have authority to install KNX products after Achieving the Certificate of E Campus

No. KNX Association does not authorise anyone to install KNX products. You need to have the correct authorisation/qualification in your country to be allowed to work on any relevant electrical systems. For example in many countries only a qualified electrician can work on “mains” 110V AC or 240V AC electrical systems.

By successfully completing the KNX Basic Certification course exams with a Certified Training Centre you can become a KNX Partner - maybe that is what you are wanting?
https://support.knx.org/hc/en-us/articles/360020923139-Advantages-of-becoming-a-KNX-partner-

Thanks Stephen, I have completed KNX  course and have ETS6 Lite version. does this mean i am qualified to do KNX projects ?

Hi Harpreet,
People who have successfully completed the 30 hour KNX Basic Certification course are given the title KNX Partner and are considered by KNX Association to have shown a basic level of proficiency with the KNX system. If you have completed the Basic Certification Course exams you should be on this list:
https://www.knx.org/knx-en/for-professionals/community/partners/index.php
If you are not on the list you have probably not successfully completed the Basic Certification course.
eCampus is not the Basic Certification course; eCampus is a precursor to the Basic Certification course.
Please search the KNX website for a KNX certified training centre in your region:
https://www.knx.org/knx-en/for-professionals/community/training-centres/