See this help page ("How to add research") for instructions how to add publications to ResearchGate: https://help.researchgate.net/hc/en-us/articles/14293005132305
Dear Suchandra Ranjit Dutta Hmm somewhat strange. What you can try is to add alternative name to your profile if you add SUCHANDRA RANJIT DUTTA besides your preferred name Suchandra Dutta. See: https://help.researchgate.net/hc/en-us/articles/14292798510993-Authorship and more specifically “How do I confirm authorship of research that I published under a different name?” for the details.
If it somehow mysteriously does not work, you can contact the RG team for assistance http://www.researchgate.net/contact or https://help.researchgate.net/hc/en-us/requests/new
Dear Suchandra Ranjit Dutta It remains a mystery. All I can think of is to wait 24 hours (perhaps the 'system' needs to adjust). It would not surprise me if let's say Sunday you can go to your profile, click on research and scroll down (left side of the page) where it states "confirm your authorship" and/or try to add the paper again as indicated in the replies above (but first wait...).
Best regards.
PS. If it is not resolved let's say on Monday, try to contact RG team. It probably will not work but you can add as alternative name, your name in capital letters.