java.lang.NoClassDefFoundError: org/reactivestreams/Publisher error

Question

I'm getting this error:

java.lang.NoClassDefFoundError: org/reactivestreams/Publisher

The code that causes the error:

Mono<Void> login = client.withGateway((GatewayDiscordClient gateway) -> Mono.empty());

I import it with this line:

import reactor.core.publisher.Mono;