Hi Rym,
The reason why you can't download CPLEX is (most probably) that you're a student, and not registered in the Academic Initiative (AI) program. The AI program is for professors and other university staff. It's up to them to register, download the software that they're interested in, and make it available to their students.
My suggestion is thus to contact your advisor, ask him to register at https://developer.ibm.com/academic/membership/, and get CPLEX for you.
Best regards,
Xavier
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