Typically, job boards consist of non-original content.  They are listings of jobs that can be found on other sites, in human resources sites, in newspapers, etc. These aren't original contents. Adsense requires their publishers to write their own contents, so if your job board contains listings that aren't "only on your site" (in other words, those jobs can't be found anywhere else except on your site), then it isn't going to be suitable for Adsense to approve.