Post Injection is a feature in AdRotate and AdRotate Pro – As described in the earlier linked page. If you don’t have those features you’re using a 6 year old version of AdRotate (Pro) or a different plugin.
For the missing adverts, maybe your adblocker is blocking them – Try disabling the adblocker.
Information, support and updates about plugins made and maintained by Arnan de Gans. Check out ajdg.solutions for more information.