You will encounter a conflict if you have Unity Ads installed in your project with the aim to integrate it with Unity Mediation, for example, to use banners from Unity Ads with Unity Mediation, or to use Unity Ads as an ad source for A/B testing performance to compare performance in different mediation providers.
Having Unity Ads as a dependency won’t necessarily break your Unity Mediation integration. However, running Unity Ads initialization code separately alongside Unity Mediation in the same session will cause runtime issues for Unity Ads inside of Unity Mediation. As a result, you will then not be able to load or show Unity Ads in both Unity Mediation and Unity Ads for that session.
The recommended solution is to remove Unity Ads from your game if you wish to use Unity Mediation.
For more information, see documentation on Unity Mediation.