Hello !
Merci à vous pour vos réponses.
En effet de le lien vers le forum d'upwork m'a fourni une réponse. Voici un copié-collé :
"If you enter a VAT number, Upwork will not charge you any VAT for its service.
If you work for EU-based clients outside of France, they may ask you for your VAT number since this is mandatory in EU. In this case, you will have to apply for a VAT number with the tax administration and they will issue one to you.
You will still remain exempt from VAT, as per French tax law, but you will have a VAT number to provide to Upwork and to your EU clients."