I contacted the support for that problem and it seems that I encountered a bug, so here’s the response that John of the Webflow support gave me :
" Hi Vianney,
Thank you for providing this information. I was also able to reproduce this on my end, and it looks like we’ve found a bug.
It looks like when there is a span element inside a field label on the checkout page it causes the field label to not be editable.
Our Product Managers and Engineers are investigating the cause of the issue and are working on a solution.
I don’t yet have a time frame for you as to when a fix will be available, but we will respond in this thread as soon as we have an update for you.
As a workaround, at the moment, you can delete the span element inside of the field label, rename the field label, and then re-add the span element with the class name.
I’ve created a short video to further clarify: https://share.getcloudapp.com/WnubOog2
Please let me know if this is helpful, or if you have any additional questions - I’m happy to help further.
Encouter the bug : add a span in field label on the checkout page cause the field label to not be editable.
Workaround : Delete the span, edit your text and re-add your span.
Maybe it will help someone later.