Copper(atomic mass 63.5 amu) has two known isotopes, cooper-63 and copper 65. explain why the atomic mass of copper is not exactly 64. ( in other words, why isn’t it midway between the mass numbers for the two isotopes

This has to do with the relative abundances (abundance=what percentage of all the element in existence is that particular isotope) of the two isotopes. The atomic mass of an element on the periodic table is given by:

(percent abundance of first isotope)*(atomic mass of first isotope)+(percent abundance of second isotope)*(atomic mass of second isotope)

Based on the mass given in the periodic table, the relative abundance of Copper-63 is larger than that  of Copper-65 because the atomic mass of Cu is closer to 63.

One other reason is that the atomic masses of the two isotopes are both less than the numbers in their names: has a mass of 64.9278 amu, and has a mass of 62.9396 amu*, so plugging these into the formula above would also contribute to the atomic mass of Cu being a value smaller than 64.

*This has to do with something called a mass defect, which occurs because of the nuclear binding effect—negative potential energy associated with attractive forces holding  the nucleus of the atom together.