In order that the Till can display any specific allergen that it may contain, you need to set that product to include an ingredient and state what that ingredient (allergen) it contains, ie. Peanuts, Fish etc.

You do this in the SumUp back office in Products>Product List

Filter and find your product that needs an allergen added to it;

In the example below we have chosen to add an ingredient (allergen) to the Cereal Bar, clicking on the pencil icon to open up the Product;

Once you scroll down to Product Inventory, click this radial button and click the 'Use Ingredient Stock' button as below and choose one or more of the offered Allergens, followed by 'Submit' to save your changes at the bottom;

Shortly afterward, you will see that when you choose this product item on the POS you will also be able to view the allergen it is associated with.