Preventatively means “in a preventative way”: you do something so that trouble will not start, rather than waiting to fix it later. You might wash your hands preventatively in flu season, or change a part preventatively before it breaks. It is a long, formal adverb. In speech, people more often say as a precaution or just to be safe. Same family argument as preventative: some people would rather say preventively, from the shorter word preventive. Both exist. The word does not mean “successfully”; it only names the timing — you acted before the problem, hoping to keep it off.