On what floor of the lost tower in Pokemon diamond is move strength on?

Move Strength is not a move that can be found in the Lost Tower in Pokémon Diamond.

The Lost Tower is primarily known for housing the Ghost-type Pokémon Gastly, Haunter, and Gengar, as well as the Ghost-type TM for Shadow Claw.

If you are looking for the move Strength, it is located in the Valley Windworks in Pokémon Diamond. You can find it inside the building where you meet the owner of the Windworks.