WordPress custom Block showing error in Posts editor but working in Pages editor

This should work:

<?php
if ( is_user_logged_in() ) {
    echo "This isn't editable in here"; 
} else { 
    include get_template_directory() . '/template-parts/blocks/form/from/functions.php';

    if ( ! isset( $_POST['firstFormDisplayed'] ) ) {
        // we haven't yet displayed the form asking for numbers, so do that now.
        displayFirstForm();
    } else {
        // we've got the numbers, but not the details, so do that now!
        displaySecondForm();
    }
}
?>