To unlock the Rare Talent trophy in Anthem, you will need to increase the Gear Score of one of your Javelin's until it reaches the Rare (blue) level. The Gear Score required is 240. Simply equipping rare items in your five slots is not enough. The overall Javelin's level must be Rare.
The gear score is an average of all of your gear. The higher the score, the higher the rarity. Once you have reached the Rare level with a Javelin, complete a mission, contract, or stronghold and the trophy will unlock at the end.