T0-220-3_Vertical is only suitable for transistors.
For LM555N, which is an 8 pin IC, kindly select the Housings_DIP:DIP-8_W7.62mm footprint. It is listed in the Housings_DIP set of footprint library.
Please follow these steps:
Open the schematic >> Go to Cvpcb >> Assign the above listed footprint to LM555N >> SAVE THE NEW ASSOCIATION >> Generate the .NET FILE >> open Pcbnew >> READ THE .NET NETLIST again.
By doing above steps, you will get the 555 IC's footprint on the layout screen.