This can happen if you do not have a Pluxee/Sodexo account. To resolve the problem:

Remove your Sodexo/Pluxee card from the Payconiq by Bancontact app via the menu on the left side of your screen by selecting the Payment methods and your Sodexo/Pluxee Card. Then choose 'Remove the card'.

Then, create a Pluxee account via the link below
Go back to your Payconiq by Bancontact app and add your Sodexo/Pluxee Card again via the menu at the top left of your screen. Select Payment methods and 'Add a card' under the heading 'Your meal vouchers'.