Hi there,
The issue is not related to the message you have. You can ignore the message as it is there because you do not have a purchase key of the Envira Gallery. But that plugin is bundled with our theme and you do not need one. For more information kindly read the bundled version section of the article below:
Regarding the issue itself, I should say that you cannot use the elements or plugins inside the off-canvas if their height is dependent on the Javascript calculation. The problem is that in the initial load the element will be hidden when it is in the off-canvas and Javascript will calculate the height 0. And that will ruin the view of it when the off-canvas gets open.
This will not happen in most modern browsers but there are plenty of them yet calculating the height 0.
So your case is one of the general rules of thumb. For example, you should not also add the gallery to a tab or accordion as those elements are hidden initially. For the tab, you can add the gallery to the first tab as it shows on load.
I hope I could explain the case in details and reason behind the Envira Gallery problem when it goes inside the off-canvas.
Thank you.