Error in the menu

Hi,
what version of Internet Explorer are you referring to?

version 11

Hello there,
it happens because IE11 does not support css variables (used by this theme and lot of other themes and plugins around the WP ecosystem).

We will think about a backward compatibility, if possible and not introducing other issues and performance issues to non IE users, in future releases.

Meanwhile you can add the following additional CSS to make it work on your website.

.main-navigation ul ul {
    background-color: #ffffff;
}
.main-navigation ul ul::before {
    border-color: transparent transparent #ffffff transparent;
}

Aside from IE11, any modern web browser supports CSS variables out-of-the-box. You can consider using Microsoft Edge as replacement of IE11, since it is bundled with Windows as well.