Bootstrap media-breakpoint-up($next) not working

Ive included in my project some template of scss mixins,variables,etc . from one repository . Guess it was this link

My navbar has the following styles

.navbar-nav {
  display: flex;
  flex-direction: column; // cannot use `inherit` to get the `.navbar`s value
  padding-left: 0;
  margin-bottom: 0;
  list-style: none;

  .nav-link {
    padding-right: 0;
    padding-left: 0;
  }

  .dropdown-menu {
    position: static;
    float: none;
  }
}

As you can see flex-direction: collumn (as it should be on mobile screen) , for responsive navbar they have written the following css property:

 @include media-breakpoint-up($next) {
    @debug $next;
    flex-flow: row nowrap;
    justify-content: flex-start;

    .navbar-nav {
      flex-direction: row;

   ....
  }

And I guess media-breakpoint-up($next) should make my navbar to be responsve and change flex-direction to row , but it is not working as I expected and it is always flex-direction: collumn. I've also debugged the $next variable @debug $next; and terminal shows sm,md,lg,xl,null and it seems like it should be correct.

Can someone help please ?