The truth is that in some cases an experienced
SEO might be able to guarantee placement for some low to medium competitive keyphrases depending on his experience, competition analysis, keyword strength, similarities with other projects he completed and several other factors.
I remember I have offered some guarantees to a few clients in the past and only once I wasn't able to deliver what I have promised so I've had to refund half of the fees. Most of the times, especially when it comes to medium to high competitive KW's, these sort of guarantees are not possible.
Beware of those companies who offer guarantees on any KW you like as this is scam and you will end up loosing money and wasting your time.
Hope this helps