To download data or products, particularly on the IDS theia.cnes.fr, you are required to have a Theia account.
The creation of this account is free and open to all.
Please visit go to the account manager on Internet: https://sso.theia-land.fr/
Then follow the steps, paying close attention to the third one.
See the enlarged image
For DINAMIS users
A Theia account also provides access to the DINAMIS data portal.
Beware! Users of Spot 6-7 images, archived or new, must have a GEOSUD account before they can have a Theia account.
The whole procedure is detailed here: http://ids.equipex-geosud.fr/signup/disclaimer