Fatal error Call to undefined function

<off-topic>
I certainly hope so. I too get frustrated at times. For me, talking out loud works.
</off-topic>

I’m guessing that the wp backery plugin interferes with the visual composer function that revslider needs.

In other words there is a compatibility conflict.

Please forgive me if my assumption that you are wanting a type of WYSIWYG because you are not up to messing with code is incorrect.

What you could try is making sure both backery and revslider are the most recent versions in hopes the issue has been resolved. If that doesn’t work you could try contacting the backery author for support or try different alternative plugins in hopes there won’t be the same problem.

EDIT
This topic is about a problem related to gutenberg. Maybe the problem you’re having is also related to gutenberg?