Hi Ichiro Sasaki,
Attempts on 1 particular certifications are limited as already mentioned by our friends here. So, the general concept is that if you fail all the attempts, so either you wait for the next release of the product & you apply on that new release and there you get again as attempt no 1, 2, 3 like that or you can apply on the similar other certifications as well similarly
Meaning, since TADM55a_75 is related to Hana with NW7.5, so in that case the certification for similarly Sybase database would be another different certification or even the certification for Hana with NW 7.4 is also different
If you are using CertificationHub I think you can not take it again using the same subscription. One subscription gives you 6 attempts summary and only 3 attempts for specific certification. If failed 3 times, it is not permitted to take it again. If I am wrong someone can correct me.