@nicolas-flam on your project you have configured your domain as an external domain
e33d4b7e-d773-4d6b-96c5-946707d6e08c-image.png
you should remove it
then you need to move your domain to the project (exactly with the screenshot you have posted above and click on the green button "Move domain"
e4d0e025-0247-433b-bd51-ed910b3ad481-image.png
once done, you will be able to activate SSL for free 🙂