In my book a digit is a character as long as it is a string.
Punctuation is more tricky here, considering the name would be a part of the URL, but I see that nobody cared about this (including me)

You have a point here, this kata is not very precise.
They could've said "a alphabetical/numerical/alphanumerical.. string of length 6"

Create a function for generating a random 6 "character" string which will be used to access the photo URL.

The kata allows to use numbers in the string.

Why? It passed the test. I don't see that rule wich is against this solution. The kata needs a 6 character long string, and the numbers are characters.

it's wrong solution, because result contains numbers only! is's against the rules of the kata.

it's wrong solution, because uuid4 contains numbers! is's against the rules of the kata.

i think need to add test case n=21,5 for example, because some variants of function returns wrong results in this case, for example this:
def test_even(n)
n.round.even?
end

if you take n=21.5 then your test_even returns true, but this is false!