Unable to Enter Blackwing Lair

Depending on which version of World of Warcraft, there could be different reasons you are not able to enter the instance. 

The Blackwing Lair raid can only be entered if you are in a raid group. If you are attempting to enter by yourself, you will receive an error message indicating that you do not meet the requirements to enter Blackrock Lair. To fix the issue, create a raid group; at least one other player will need to be in the raid group before you can enter.

In Classic Era, you can only enter Blackwing Lair if you first complete the attunement process. If you are unable to interact with the orb in front of the instance, it is because you have not completed the attunement. You can find details for the attunement process here.