Top Four Reasons Why You May Receive a Serial Number Error During Activation of Revit

1. Typos are made during activation
2. The serial number you are attempting to use does not match the product
3. The serial number you are attempting to use is truly not valid anymore
4. The contact information you are using does not match what is already associated with the Serial Number when the product was registered

From Autodesk – Autodesk Revit Architecture Services & Support – Top Four Reasons Why You May Receive a Serial Number Error During Activation