A.L.T.E.R. is an Uncommon Fortnite Spray. A.L.T.E.R. was available via the Battle Pass during Season 11 and could be unlocked at Tier 51.
How-to Get the A.L.T.E.R. Spray
A.L.T.E.R. is part of Season 11. If that season is still currently in the game, you can obtain this item by purchasing and/or leveling up your Battle Pass.