How to write if else statement that loads image from custom field in WordPress else loads a place holder image if no custom image is chosen

Use the following and see what it prints out:

<?php
                  foreach( $myposts as $post ) :      setup_postdata($post); ?>

                  <? if ( get_post_meta( get_the_ID(), 'wpcf-header-image', true ) ) {
                        $store_imgs = get_post_meta($post->ID, "wpcf-header-image", true);
                                var_dump($store_imgs);
                  }  else {
                        $store_imgs = "/wp-content/uploads/2018/12/Products_Banner_BG800px-tall-2.jpg";
                  }
                  ?>      
                                                                  
<div class="slide-img" style="width:100%; height: 800px; background-image: url('<?php echo $store_imgs; ?>'); bac

So I tried the code provided I ended up getting a 502 error 

Try your code again but with the Var_Dump, like this and let me know what is printed:

<?php
                  foreach( $myposts as $post ) :      setup_postdata($post); ?>

                  <? if ( get_post_meta($post->ID, "wpcf-header-image", true) ) {
                        $store_imgs = get_post_meta($post->ID, "wpcf-header-image", true);
echo '<pre>';
var_dump($store_imgs);
                  }  else {
                        $store_imgs = "/wp-content/uploads/2018/12/Products_Banner_BG800px-tall-2.jpg";
                  }
                  ?>      
                                                                  
<div class="slide-img" style="width:100%; height: 800px; background-image: url('<?= $store_imgs; ?>'); background-position:center center; background-size:cover;">
</div>

I think there is something wrong with this code - It breaks the site - So First I need the code not to break the site before I can tell you what is outputted - I get the the same 502 error as before using the code you had in the above suggestion