I’ve taken a look at your site and noticed that it uses a library called jquery-migrate.
WordPress 5.5 has removed that library and plugins/themes that still use it should add it themselves. WordPress announcement, more context.
One thing you can do to update your site to WordPress 5.5 is to install this plugin https://wordpress.org/plugins/enable-jquery-migrate-helper/ that adds back the library.
Thanks for that, I missed that announcement. You are a stAR.