Facing a problem with Social Security registration ?

Hello,
I'm an international student. I tried to do Social Security registration for foreign students on https://etudiant-etranger.ameli.fr/#/. However, I received an e-mail saying:

"Nous ne pouvons donner suite à votre demande d'inscription car votre situation ne le permet pas : Un numéro d'immatriculation vous a déjà été attribué.
Nous vous invitons donc à prendre contact avec la caisse du lieu de votre résidence.
Votre correspondant de la sécurité sociale étudiante."

I used to work as an intern in 3 months in France from mid-April to mid-July in 2018 then started a new study program in another city in France. I didn't register for Social Security at that time. According to what I already read, the Social Security number might appear on my payslips (there could be someone else already doing the registration for me). However, I don't have them anymore since it's been a long time and my working e-mail at that time has no longer been available to access since September. What can I do to find or get my Social Security number back ?

Thank you very much.