Sorry to hear about the problem you are having.
I have tested it on my test site and it’s working fine for me using the latest version of the Jevelin theme as displayed in the attached screenshot.
Please make sure that you are using the below latest version of the theme and try again.
Please login to access this file
Would you mind if I log in to your site and do quick troubleshooting? If this is ok then could you please temporarily create an admin user account and share the account login details privately by adding them in the box having text “Enter your private content here (only you and forum moderators will be able to see it)”?
I logged into your website and now can not even access the Pages section in the admin area of your website and get an error as shown in the attached screenshot.
It is your website-specific error that you can resolve as described below or by contacting your web host.
Please note that you don’t need to activate all the recommended theme plugins on your website but just the required plugins.
If you have upgraded to the Redux framework then you have to deactivate the Unyson plugin on your website.
how to fix this problem
It is not a problem but a notice and you can remove it by following the instructions provided in the notice.
and can you share more videos about this template. how to use? how to edit? connect with form ?
There are no videos.
You can edit the page using the WPbakery page builder as displayed in the attached screenshot.