I had the same problem described in this thread https://Formidableforms.com/help-desk/bootstrap-modal-and-popup-forms/.  My problem was caused by multiple versions of bootstrap being loaded. I solved my problem by adding the following code to my child theme's functions.php (as always, make sure you have FTP access to your site before editing functions.php from the WordPress backend in case something goes wrong):

add_action('wp_footer', 'remove_bootstrap_js', 1 );
function remove_bootstrap_js(){
 wp_dequeue_script( 'bootstrap' );
 wp_dequeue_style( 'bootstrap' );
}