public operator fun DependencyAdder.invoke(dependency: Provider<out Dependency>): Unit defined in org.gradle.kotlin.dsl
public operator fun DependencyAdder.invoke(externalModule: ProviderConvertible<out MinimalExternalModuleDependency>): Unit defined in org.gradle.kotlin.dsl
```

To fix this, replace the reference to `project` with a call to `project()`:

```kotlin
testing {
  suites {