Jelly, 8 bytes
ÆNḃ26ịØAÆNḃ26ịØa
A monadic Link which accepts a non-negative integer that yields a list of characters.
How?
ÆNḃ26ịØAÆNḃ26ịØa - Link: integer, n
ÆN - nth prime
26 - literal 26
ḃ - bijective base
ØAØa - literal list of characters = ['A'['a', 'B''b', ..., 'Z']'z']
ị - index into