Weekly and Nightfall Strikes are, this week, off the rotation for non-DLC buying players in Destiny. Today saw the launch of The Dark Below - the first expansion for Bungie's shooter - and as you might expect the new content is up front and centre, with the Weekly and Nightfall Strikes currently DLC-only missions.
That means, if you don't have the expansion, you won't be able to play these missions and thus earn the additional loot that drops with them. However, next week that'll change, when different missions replace the old, and all Destiny players will once again be able to get back to grinding through these highlighted missions as they seek to upgrade their gear (that is until the new missions are back on the rotation, whenever that may be).