If you integrate UnityAds directly without any mediation (aka Unity Advertisement Legacy package), it is normal behavior not to be able to see the Placement ID section in your dashboard because you only need Ad Units.
However, if you integrate UnityAds through any mediation but cannot see the Placement ID section in your dashboard, you should go to the Dashboard> Monetization > Choose the project on the top > Settings > Mediation Partner and choose the appropriate mediation partner.
After doing this you should be able to see the Placements menu.
Please refer to this guide for more information on Placements and eCPM targets.