Why Element h2 not allowed as child of element span?

Hello!
When I check my site, this error occurs:
"Error: Element h2 not allowed as child of element span in this context.
(Suppressing further errors from this subtree.)"
Why is H2 not allowed as a child of element span?! What is the reason?
What element should I use instead?

Thanks & Regards,
Admin of https://newvisiona.com