Not True
There is no vitamin named B17 - the complex of B vitamins ends at B12.
What was called B17 is in fact a chemical called Amygdalin or Laetrile, found in a number of plants, and not naturally in the human body.
No reasonable study had shown that Amygdalin/Laetrile or related products have any effect on tumours - indeed there are instances of people poisoning themselves with the substance.
Cancers themselves are caused by a number of factors; the process itself is of cellular DNA mutation leading to cells that grow uncontrollably.
Unfortunately the Vitamin B17/Amygdalin/Laetrile thing is a pretty good example of quackery; people selling an ineffective, unproven product based on a fabricated premise. And ultimately, people end up being harmed, either from poisoning, or from neglecting appropriate treatment until it’s too late.