I was copy-pasting the example and it wasn't working. Then I found that the example in the comment was a typo.
Thanks
// Navbar vertical align
// -------------------------
// Vertically center elements in the navbar.
-// Example: an element has a height of 30px, so write out `.navbarVerticalAlign(30px);` to calculate the appropriate top margin.
+// Example: an element has a height of 30px, so write out `.navbar-vertical-align(30px);` to calculate the appropriate top margin.
.navbar-vertical-align(@element-height) {
margin-top: ((@navbar-height - @element-height) / 2);
margin-bottom: ((@navbar-height - @element-height) / 2);