<Ø[₂‰˜0KZ27‹#}<Asè
Isn't there a convenient builtin to do this shorter?.. I have the feeling I should be able to use one of the base-conversion builtins here, but it isn't really working out.. Can without a doubt be golfed substantially, though..
Try it online or verify some more test cases.
Explanation:
< # Decrease the (implicit) input-integer by 1 to make it 0-based
Ø # Get the 0-based n'th prime
[ # Start an infinite loop:
₂‰ # Take the divmod 26
˜ # Flatten the resulting list
0K # And remove any 0s
Z # Get the maximum of the list (without popping)
27‹ # If it's smaller than 27:
# # Stop the infinite loop
}< # After the loop: decrease all values by 1 to make it 0-based
A # Push the lowercase alphabet
sè # And index the list of integers into it
# (after which the resulting character-list is output implicitly as result)