HttpMessageNotWritableException no converter for class

Question

I have this mock controller:

    @GetMapping(value = "/mock", produces = MediaType.TEXT_PLAIN_VALUE)
    public ResponseEntity<BodyResponseDTO> getMock() {
        try {
            BodyResponseDTO bodyResponseDTO = new BodyResponseDTO();
            bodyResponseDTO.setCode(HttpStatus.OK.toString());
            bodyResponseDTO.setMessage("Risposta OK");

            return new ResponseEntity<>(bodyResponseDTO, HttpStatus.OK);
        } catch (Exception e) {
            log.error("Errore: ", e.getCause());
            return new ResponseEntity<>(HttpStatus.INTERNAL_SERVER_ERROR);
        }
    }

the exception is: Resolved [org.springframework.http.converter.HttpMessageNotWritableException: No converter for [class com.consumer.message.BodyResponseDTO] with preset Content-Type 'null']

Keep in mind that I build the http headers in this method:

    private HttpHeaders buildHttpHeaders(HeaderRequestDTO headerRequest) {
        HttpHeaders headers = new HttpHeaders();
        headers.set("Accept", headerRequest.getAccept());
        headers.set("Agid-JWT-Signature", headerRequest.getAgIDJWTSignature());
        headers.set("Authorization", headerRequest.getAuthorization());
        headers.set("Content-Type", headerRequest.getContentType());
        headers.set("If-Match", headerRequest.getIfMatch());
        headers.set("Content-Range", headerRequest.getContentRange());

        return headers;
    }

and the Accept headers is often if not always "text/plain"

How to solve this exception? Thanks in advance