The referee said my attack was 'preparation', what does that mean?

It means the referee decided you weren't attacking yet, you were getting ready to attack, when your opponent started theirs. And an actual attack beats a preparation, so their touch counted and yours didn't.

'Preparation' covers everything you do before your final action: stepping forward without your arm extending, pumping the blade, searching for their blade, a feint you didn't convert. The moment your advance stalls or your arm pulls back, a sharp opponent attacks INTO that moment, and the ref sees their attack starting while you're still preparing. Their priority.

How to stop hearing this call: finish. When you step forward, your arm goes with you, extend as you close distance, so what the referee sees is one continuous attacking action, not a walk followed by a poke. If you like to prepare with your feet (many fencers do), keep the prep short and be honest that during it you're vulnerable, either be ready to parry the attack-into-prep, or make your prep itself a threat.

It's genuinely one of the most argued calls in fencing, even top international bouts turn on attack-versus-preparation. If a referee keeps calling it on you, they're giving you free coaching: your attack is starting too late.