flex space-between doesn't work

You are looking for justify-content: space-between.

Updated Example

MDN justify-content

The CSS justify-content property defines how a browser distributes available space between and around elements when aligning flex items in the main-axis of the current line. The alignment is done after the lengths and auto margins are applied, meaning that, if there is at least one flexible element, with flex-grow different than 0, it will have no effect as there won't be any available space.

a {
  text-decoration: none;
  width: 98px;
  height: 40px;
  display: flex;
  flex-flow: row no-wrap;
  align-items: center;
  justify-content: space-between;
  background-color: lightgray;
}