Yes, I already knew the cause of what you're experiencing:

The $page and $pages global variable have not been set up and a call to get_the_content or get_the_excerpt would return that error: count(pages)

That said, pages global variable is set to null, calling count(pages) triggers that error because a null value cannot be counted.

To fix this, one can easily tell that you're calling get_the_content or get_the_excerpt outside the loop.

So you have to set up the post data correctly before usage, that's what the loop is meant to do:

while ( have_post(): the_post() ) :
    // get_the_content() or get_the_excerpt() will work fine now
    the_content();
endwhile;

As of v5.2, this will no longer be an issue. get_the_content() and wp_trim_excerpt() are getting an optional new parameter (third and second respectively) of $post so you can use it outside the loop, bringing it in line with the functionality of all the other get_the_* functions. It only took 16 years... Reference on Trac

If someone bumps in to this issue, the solution is to get the_content from the meta field and run it trough the_content filter like this:

apply_filters('the_content', get_post_field('post_content', $post->id));

I made a short video explayining the whole thing if you wait untill the TLDR section... https://www.youtube.com/watch?v=vMguTNzFoUk

This bug related to WordPress core bug https://core.trac.wordpress.org/ticket/42814

Most often it occurs when you call the function get_the_excerpt or get_the_content outside the Loop provided that excerpt or content is empty.

So to fix it yourself you should find error place and check excerpt/content existing manually.

For example:

if(has_excerpt($post_id)) {
    $meta[] = sprintf('<meta name="description" content="%s">',
        get_the_excerpt($post_id)
    );
}