Welcome to Eorzea!

Any class/Job is 'viable' for group content, regardless of what the 'meta' may have you believe. It's just DRK is more complicated than the other tank classes/Jobs (GLA/PLD is a perfect novice tank that is simple to play as and master, while MRD/WAR is the 'meta' tank that has a slightly more complex mechanic to it but deals lots of damage while being able to heal itself. DRK has it's own mechanics that may be more daunting to a new player, but I must say their level 30-50 Job quest storyline is my favourite Job quest arc in the entire game, but I won't spoil anything more.)

By the time you're able to unlock DRK when you reach Ishgard upon starting the Heavensward story, you should have a firm grasp of tanking in general to try out DRK yourself and if you find you enjoy it, go for it! Play a Job you want to play as, a tank will always find a party regardless if you're a DRK or another class/Job.