Can´t create a pricing using extension fields

Hi experts

Actually I´m triyng to configure a pricing by extension fields, just as mentioned in "Pricing with Extension Fields" help center document, but when i reach step 3 in example 2, the following error message is shown, so i can´t cofigure the discount list.

Does anyone knows the correct process?

So this example is with list fied, anyone knows if i can do it with an import field?

Best regards.