Pectin is crucial for the recipe. It is what helps the jelly thicken and gets it to that spreadable texture you're looking for. However, if you don't have it and are looking for an alternative, you can add a bit of cornstarch into the mix and let it simmer for a minute or two until the mixture thickens. Alternatively, you can soak a couple of gelatin sheets in water and add them into the jelly towards the end while it is still hot, but not scalding hot.