syn request in webclient cannot use block for Mono in springboot - spring-boot

Question

I am new to reactive and web client and i just doing a project in springboot microsevice.While when setting the ReactiveUserDetail Service which require the use of mono.I find that my synchronous request by webclient give an error.I suspect this is due to the "block" ,but it seems that i cannot eliminate it.
Error:
java.lang.IllegalStateException: block()/blockFirst()/blockLast() are blocking, which is not supported in thread reactor-http-epoll-3
at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:83) ~[reactor-core-3.5.1.jar:3.5.1]
Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException:
Error has been observed at the following site(s):
*__checkpoint ⇢ org.springframework.security.web.server.authentication.AuthenticationWebFilter [DefaultWebFilterChain]
*__checkpoint ⇢ org.springframework.security.web.server.context.ReactorContextWebFilter [DefaultWebFilterChain]
*__checkpoint ⇢ org.springframework.security.web.server.header.HttpHeaderWriterWebFilter [DefaultWebFilterChain]
*__checkpoint ⇢ org.springframework.security.config.web.server.ServerHttpSecurity$ServerWebExchangeReactorContextWebFilter [DefaultWebFilterChain]
*__checkpoint ⇢ org.springframework.security.web.server.WebFilterChainProxy [DefaultWebFilterChain]
*__checkpoint ⇢ HTTP POST "/signin" [ExceptionHandlingWebHandler]
Original Stack Trace:
at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:83) ~[reactor-core-3.5.1.jar:3.5.1]
at reactor.core.publisher.Mono.block(Mono.java:1710) ~[reactor-core-3.5.1.jar:3.5.1]
My code:
#Service
public class SecurityUserService implements ReactiveUserDetailsService {
#Autowired
private WebClient.Builder webClientBuilder;
#Override
public Mono<UserDetails> findByUsername(String username) {
UserAuthdto result= webClientBuilder.baseUrl("http://USER").build().get() //make syn request
.uri(uriBuilder -> uriBuilder
.path("/User/AuthUser/{username}")
.build(username))
.retrieve()
.bodyToMono(UserAuthdto.class)
.block()
;
//map the userresult into securityuser
if(result!=null){
return Mono.just(new SecurityUser(result));
}
//no userfound
return null;
}
}
Entry point from the other microservice:
#GetMapping("/AuthUser/{username}")
public UserAuthdto getSecurityUser(#PathVariable String username){
return userCoreservice.getSecurityUser(username);
}
Web client bean:
#Bean
public WebClient webClient(){
return WebClient.builder().build();
}
Please help