User blog comment:Ssj3gogeta96/Ways to kill a soul reaper/@comment-5398287-20130601080932

Well, I'm not sure why you're referring to statements about how Shinigami die when you're asking about why Ichigo couldn't cut Kenpachi, but I'll answer both questions.

1. It takes a lot to kill a Shinigami, but it's not impossible. They can be killed by being decapitated/having their head crushed (Zommari states the former, the latter happens during the Vandenreich's invasion when Buzzbee crushes the head of a Shinigami underfoot), having their body destroyed (Tosen's body exploded, Yamamoto's was obliterated), or taking severe injuries (Gin had his arm torn off and a hole opened in his body, I'm not gonna get into an argument about whether or not Ichigo died from having a hole blown through his chest by Ulquiorra but that's the kind of injury that will kill a Shinigami).

2. Ichigo was unable to cut Kenpachi at first because his Reiatsu was lower than Kenpachi. Essentially, if the Reiatsu of being A is not as high/concentrated/controlled as the Reiatsu of being B, being A can't harm being B because they're essentially on different levels. As for Urahara, he does have a real Zanpakuto, and back then Ichigo was still learning about fighting and Shinigami and stuff, so he didn't really understand it.

Hope that helps.