In order for someone to be able to "channel" Reiki (connect to and use the energy that is Reiki) they must have an attunement - a sacred and secret ceremony performed only by a qualified Reiki Master Teacher, in whatever modality of Reiki you choose be it Karuna Reiki®, Seichim Reiki, Usui Tibetan Reiki, or in the original form of Reiki - Usui Reiki, etc.
Worknik definition of an attunement: n. The quality of being in tune with something
In many Reiki traditions, there are a minimum of three levels (four in mine) of training that one has to take in order to became a Reiki Master/ Reiki Master Teacher. Each new level of training requires that you have another attunement. Each attunement imitates a 21 day detox (possible 30 day) or cleanse in the 7 major chakras of the body. Each persons response to each detox/cleanse is different and individual, and each level of attunementss detox or cleanse is different and individual.
I recently read a blog post where someone said that an attunement initiates a "healing crisis" not a 21 day cleanse. In all the books I've read on Reiki (over 60) and as taught by both of my Reiki teachers, that is not the case. One person out of thousands might have a more intense cleanse if they do something stupid like I did and take all three levels of training in one weekend (I didn't know what I was doing and had "drive through" Reiki as I like to call it). There is always an exception to every rule!
The attunement connects the individual to the source of the Reiki energy so that they can channel it for themselves and others. Without it, you cannot connect to Reiki period. There are many people and websites offering "free" or low cost Reiki attunements on the internet. My take on that is if you are living out in the "boonies" and you and there are no Reiki Master Teachers available to you for hundreds if not thousands of miles away, or you have such limited finances that there is just no way you can afford the training, and you have a very strong desire to be become Reiki, then this maybe your only alternative, albeit a poor one.
The advantages of having your attunement performed live and having access to that person for continued education, possible treatment (some people require a Reiki session or two after an attunement to help them get through the cleanse easier), and providing community through Reiki Shares, far outweighs the benefits of distant attuning. How do you really know that the person claiming to attune you is actually doing anything at all besides taking your money?
I recommend that you interview several Reiki Teachers and have sessions with each one to see who you resonate with. Each teacher has unique qualities that they bring to their practice and someone might not "feel" like they are the person you want to attune you and mentor you on your Reiki journey, and someone else might feel "just right." I believe the Universe or Spirit will guide you to the perfect teacher for you.