You might have faced a tricky situation where after completing the new story missions of Destiny Dark Below, you would be prompted to complete the new quests when you want to play the weekly Nightfall / Heroic.
Either way, we have got you covered; just follow the steps explained below and you will be able to get what you want.
How to Unlock Weekly Nightfall / Heroic
The first thing you want to do is to go to Eris in the Tower and get the first quest. Don’t know where she is? Well, go left from the place where you spawned and you will find what you are looking for.
The quest asks you to put down 25 Knights or Wizards. One way to do this would be to and revisit the second DLC quest that you play on earth. You might have to delay killing the main enemy in order to get more wizards and knights come at you; you might also have to do it more than once.
Secondly, you head to Eris again and get three new Patrol quests. On Earth, after you have chosen Patrol, you should go right from where you are spawned for the first location. Keep going through the facility, cross the Divide and go past the Rocket Yard like you would go to the Sepiks Prime strike.
Once you are past the Rocket Yard, look for a Hive enemy on the left. Confront and kill it along with all the enemies that spawn later.
For the second Patrol quest, you should head back and return to Earth and to Patrol. This time go straight towards the place where you kill the first Wizard; inside the complex over the hill.
As you draw close to the door of the room where you killed the Wizard, you will find a Thral. Killing it and everything else in the room gets you the second success.
Keep going forward for the third quest (no need to reload like the second one). Go past the wizard room and towards the original loot cave. On thee right side of the cave, there is a building; go inside and get to the opening of Terrestrial Complex.
Go further still and into the small landing; in there you can steal the Urn before killing all the enemies. Once you are done with all this, your weekly Nightfall and Heroics will be unlocked.
If there is anything missing or confusing in all this, let us know so that we can help you.