In order to add a variable-price product you must follow the steps below:

1. Go to your BackOffice > Configuration > You modules > Order Taking Settings and activate the module "Variable product price". 

2. Then in the BackOffice go to Products > products and whenever you create a new product or select one, already created you will have the option "variable price", activated and click on > Edit.  

3. Remember to refresh your App

4. Now, each time you add a variable product price to an order, a pop-up will appear to allow the user to type the variable price, then click on validate.