Invalid document (Proof of registration for the current academic year)

Hello,

I am a Ph.D. candidate from Thailand. I obtained a student visa and visited France as an exchange or visiting Ph.D. student. I did not enroll in French institution, but I have a hosting agreement with the university that confirms that I will conduct research in France as a Ph.D. student for 8 months. I attempted to apply for social security several times but was denied due to issues with Proof of Registration for the current academic year. Previously, I attempted to attach an invitation letter and a hosting agreement but was rejected. Could you please assist me in resolving this issue or advise me on how to obtain a social security number and a carte vitale?

Thank you in advance for your help.