You probably use this in your project:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-webflux</artifactId>
</dependency>
in which case you'll have to also add:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
and the magic happens :)
PS: that's because Spring will use by default web-MVC instead of web-flux when both are available