@JstantoineWhat browser are you using? Chrome or firefox work best. Have you gotten that message from the very beginning? There is a chance you have a faulty sim if you got the invalid sim message on your very first attempt.
I am using chrome I've tried my other browser with same results. It worked the first time just the final part of the activation failed so the sim is not faulty
@Jstantoine Did you finally have success? As posted earlier clearing your cache/browser and using incognito/secret mode is essential. Continue to do this when making any account related changes such as payments, credit card or plan changes and adding/purchasing add ons.
@Jstantoine Are you still getting the invalid sim message or are you getting a random error message during activation and then having to wait out the lockout session?