If "Add Valid Host Name" violation is detected, should I accept or delete it. Sufficient sample request and score level is 100.

If there are numerous other violations associated with the request, such as attack signature violations within the host name itself, or HTTP compliance failures within the request, then the suggestions can be deleted and you can wait to see if they occur repeatedly before making the decision. If you know that the host name is valid, then it's safe to manually accept a suggestion even if the learning score has not yet reached 100 percent.