Review the info on the link to the ValveWizard site, there is a good explanation of an artificial center tap there, with options. Basically, there needs to be a ground reference for the 6.3v filament (heater) circuit to provide hum cancellation. Some transformers include a center tap for this purpose, the P-TF22772 does not…no big deal though because you can create an “artificial center tap” by adding a 100r (NOT 100k) resistor to each side of the circuit. If you are building your amp like the book (or kit instructions), the pilot lamp holder is a convenient place to add the resistors. You have the two green wires (the 6.3v taps) going from the PT to the pilot light and then on to V2 and V1, simple add a 100r resistor, going to ground on each one of the taps where they connect to the pilot lamp holder and you’re good to go. BTW, adding the ground reference to the heater circuit is VERY important in terms of hum cancellation.
Some transformers come with center taps for other secondary pairs coming form the PT, unless the schematic calls for these to be connected, DO NOT assume they should go to ground. Simply cap them off and leave them unconnected. With your transformer, only the RED B+ (high voltage) taps should have their center tap (Red and Yellow)connected per the schematic.