Well, these instructions are not clear enough and could be updated with the example given by you to make it understandable.
Thank you, anyway.

For example if passed "Hannah" it should return 6

That alone should tell you the letters in the palindrome are not unique.

$aaabbbccddd_!jJpqlQx_.///yYabababhii_ -> aaabbbcjqyidddiyqjcbbbaaa 25 chars.

How comes that the result for "$aaabbbccddd_!jJpqlQx_.///yYabababhii_" should be equal to 25, if the count of the unique alphanumerical chars for this string is equal to 12?
How comes that the other expected results are way larger than the length of the max possible palindrome from an alphanumerical string (36*2)?

Love this one!
Feeling so dumb, especially when seeing this:

letter.toLowerCase() <= 'm'

Examples works fine, but suite returns this:

Passed: 0 Failed: 0

STDERR:
solution: virtual void POSTFIX::TESTS(): Assertion `to_postfix(infix) == postfix' failed.

You have received this error?
If yes, I've got the same problem.

// solution: virtual void POSTFIX::TESTS(): Assertion `to_postfix(infix) == postfix' failed.