Issue:: Getting 500 internal server error when trying to use the put and post methods on postman to my spring rest app.

Tell me the possible solution so that i can follow it up , i am beginner so plz tell me in more organize way and step wise step..

//For more clearity, see this line 6 message information::

Type definition error: (simple type, class com.ukg.shared.models.KeyVal); nested exception is com.faster..jackson.databind.exc.Invalid DefinitionEx ception: Cannot construct instance of com.ukg.shared.models.KeyVal (no Creators, like default constructor, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information at [Source: (PushbackInputStream); line: 4, column: 13 1 (through reference chain: java.util.ArrayList[0]->com.ukg.communication.sms.models.MessageBucket{\"keys\"\->java.util.ArrayList[0]) Postman - X File Edit View Help Home Workspaces Reports Explore Q Search Postman Sign In Create Account Working locally in Scratch Pad. Switch to a Workspace X 6 Scratch Pad New Import L. POST (. POST LO POST (. POST I. GET L. + 000 CD-REC (Local] N + = 000 0 Campaign Management / Dev Controller / Load Messages for Blasts I Save V 000 Collections CD-REC E POST Send V Campaign Management {{Campaign Management Base URL}}/api/sms/v1/dev/load/messages APIs > Remote Services Controller Params Authorization Headers (12) Body Pre-request Script Tests Settings Cookies > > Campaign Controller Body Cookies (1) Headers (12) Environments Test Results 500 Internal Server Error 53 ms 1.27 KB Save Response V Dev Controller POST Gen Pii for Tenants Pretty Raw Preview Visualize JSON Q Mock Servers POST Load Pii for Tenants 1 { POST Gen Blasts for Tenants 2 3 4 Monitors POST Load Blasts for Tenants 5 POST Gen Messages for Blasts 6 History POST Load Messages for Blasts "timestamp": "2022-01-07T13:07:33.153+00:00", "status" : 500, "error": "Internal Server Error", "exception": "org.springframework.http.converter.HttpMessageConversionException", "message": "Type definition error: [simple type, class com.ukg. shared models. KeyVal]; nested exception is com.fasterxml.jackson.databind.exc. InvalidDefinitionException: Cannot construct instance of .com.ukg. shared models.KeyVal (no Creators, like default constructor, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information at [Source: (PushbackInputStream); line: 4, column: 13] (through reference chain: java.util.ArrayList[0]->com.ukg.communication.sms.models. MessageBucket [\"keys\"]->java.util.ArrayList[0])", "path": "/api/sms/v1/dev/load/messages" GET Build campaigns (genera... POST Load campaigns DEL Clear campaigns POST Load recipients 7 a Find and Replace e Console Runner Trash