JsonMappingException: Can not refine serialization key type in Spring Boot

My Spring Boot Application is trying to convert Date field in a POJO Class to String, but while conversion, I get below error-

com.fasterxml.jackson.databind.JsonMappingException: Can not refine serialization key type [simple type, class java.util.Date] into com.fasterxml.jackson.databind.ser.std.JsonValueSerializer; types not related (through reference chain: ...)
        at com.fasterxml.jackson.databind.AnnotationIntrospector.refineSerializationType(AnnotationIntrospector.java:891)

MyTransactions.java

import com.fasterxml.jackson.databind.annotation.JsonSerialize;
import com.fasterxml.jackson.databind.ser.std.JsonValueSerializer;
import java.util.Date;

public class MyTransactions {
  private String id;
  private String modifierId;
  @JsonSerialize(as = JsonValueSerializer.class)
  private Date lastUpdatedOn;
  private String amount;
}

I am trying to use Jackson to serialize Java class to JSON String. What is the issue and how to resolve it?