Responsiveness with Grid/Flexbox, mobile menu with Vanilla JS

Hi guys,

I was trying to use BEM standarts for naming css classes. Am I doing it right?

I was also trying to make it responsive on all screen sizes, but it ended up with a little messy code.

I dont understand why I have HTML error in:
<li> elements must be contained in a <ul> or <ol> when according to MDN <menu> should be subtitute for <ul>

Any suggestions welcome!