Elderberries are very low in pectin, so you can add some apples and lemon juice that are high in pectin to help thicken the jam. This recipe is made without pectin and that is why it is important to incorporate those ingredients that are going to provide enough natural pectin to create an even and enjoyable texture. If you are, however, considering to rather add pectin to your jam, check out this article that helps you understand the benefits of using pectin.