Not an issue

You probably just need to replace puts h with h. The return value of puts is nil.

The first test i see is
Test.assertEquals(capital(state_capitals)[0], "The capital of Maine is Augusta");

this does not seem correct, shouldn't it be
Test.assertEquals(capital(state_capitals[0]), "The capital of Maine is Augusta");

As I understand it you are passing in a single object from the array, the first (incorrect) test will output only the first letter of the returned value,