guest post is content that you publish on someone else's website, and it can, or can't include a link that points back to your site (so, a backlink), depending on whether you want to include the backlink in that guest post (which you want to do, obviously, that's the only reason you're paying for that guest post), or whether the owner of the website giving you the guest post allows the inclusion of links pointing back to your site.
But if the owner of the site that publishes your guest post doesn't let you have a backlink inside of that guest post then you're being scammed 100%, because the only purpose of guest posting on someone else's website is to get a backlink from them so you can increase your site's authority (DR / DA / strength / whatever you want to call it) so that you can rank higher in google because google favors websites that - among other things - have many quality backlinks.