Guide from gamerankx below as the above guide did not mention the name of the exact mission that you need to complete.
To unlock the side-mission ‘A Merry Chase’, go down the hallway opposite the Janitor’s Room in the Janitor’s Office area, near the start of the Maintenance Sector. Go down the hall and you’ll find a break room with a cache inside. Go for the cache, and the floor will crumble — dropping you into an Astral Plane challenge.
Here, you’ll learn about the ‘Evade’ ability. This is completely optional and can be missed. The ‘Evade’ is a quick-dodge, allowing you to avoid explosions, attacks, or close the gap and strike enemies with your melee attack. Evade cannot be upgraded, but it’s also useful for traversing the environment before you unlock the ability to Hover.
Complete the Astral Plane challenge to unlock ‘Evade’ — it’s a very handy ability, and you’ll want to grab it as soon as possible.