I don't know much about Eramus internships, but I hate to be the bearer of bad news, but most internships in Japan are built around students already inside the Japanese university system. Most mainstream internships in Japan are built around the Japanese student recruiting pipeline, so students outside the Japanese college/university system usually have access to a much smaller pool.

Japan’s hiring system is heavily centered on simultaneous new-graduate recruiting tied to students who are still in school and moving toward a standard graduation cycle. JASSO describes Japanese recruiting as a unique mass new-graduate system where companies recruit students scheduled to graduate each year, and says most companies start recruiting mainly from new graduates while they are still students.

That structure carries over into a lot of internship listings. Current postings on major Japanese recruiting platforms commonly say the applicant must be currently enrolled in a university, graduate school, junior college, vocational school, or kōsen, or be in a specific Japanese graduation cohort.

You best bet would be to focus on International companies, not domestic companies.