You need to include reactor-netty on the classpath. As by default that is the HTTP client used by WebClient.

pom.xml:

<dependency>
    <groupId>io.projectreactor.netty</groupId>
    <artifactId>reactor-netty</artifactId>
</dependency>

More info: https://docs.spring.io/spring/docs/current/spring-framework-reference/web-reactive.html#webflux-client

Tested converting an existing integration test that was using TestRestTemplate into integration test using WebTestClient

package no.mycompany.myapp.user;

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.reactive.AutoConfigureWebTestClient;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.web.reactive.server.WebTestClient;

@AutoConfigureWebTestClient
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
public class LoginControllerTest {

    @Autowired
    WebTestClient webTestClient;
        
    @Test
    public void postLogin_withoutUserCredentials_receiveUnauthorized() {
        webTestClient.post()
                .uri("/login")
                .exchange()
                .expectStatus().isUnauthorized();
    }
}

Added this to POM

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-webflux</artifactId>
        <scope>test</scope>
    </dependency>

The following was already in POM

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>