Hi Sayontan,
Thank you for reaching back out. Unfortunately, the error message is on the front-end. It looks from what I can find like this theme has a default gallery built into the theme, which somehow doesn’t allow some other gallery plugins to function correctly. It looks like many people have had this issue & it is usually resolved by them going into the user’s backend and changing the coding themselves. Sadly, they don’t seem to be as responsive as you.
Thank you so much again. I sincerely appreciate you going above & beyond to try to help.