Which two letters are not present on the English Periodic Table of the Elements?

The standard notation for an element is an upper case letter followed sometimes by a lower case letter and enclosed in a square. That is an image that cannot be represented here in normal typeface. Many single letters represent the most basic elements, Oxygen {O}, Hydrogen {H}, Carbon {C}, Nitrogen {N} and so forth. Some elements need an additional latter to make a distinction such as {Xe} for Xenon, and {Ir} for Iridium, but only {V} for Vanadium only {K} for Potassium.

There is no reason that the letters J and Q are not allowed to be included, but the namers of the elements make that distinction. A previous question here incorrectly lists only J.