How to fix the ValidationException with AWS amplify mock api - amazon-web-services

Question

I get the exception ValidationException: One of the required keys was not given a value
when I click on the Play button as instructed in an AWS workshop page. Any advice would be really helpful!
AppSync Mock endpoint is running at http://192.168.20.3:20002
Error while executing Local DynamoDB
{
"version": "2018-05-29",
"operation": "PutItem",
"attributeValues": {
"type": {
"S": "post"
},
"content": {
"S": "test"
},
"timestamp": {
"N": "1668425585"
},
"owner": {
"S": "7d8ca528-4931-4254-9273-ea5ee853f271::user1"
},
"__typename": {
"S": "Post"
}
},
"condition": {
"expression": "attribute_not_exists(#id)",
"expressionNames": {
"#id": "id"
}
},
"key": {}
}
ValidationException: One of the required keys was not given a value
Schema file
input AMPLIFY {
globalAuthRule: AuthRule = { allow: public }
} # FOR TESTING ONLY!
type Post
#model(
mutations: { create: "createPost", delete: "deletePost", update: null }
timestamps: null
subscriptions: { level: public }
)
#auth(
rules: [
{
allow: owner
ownerField: "owner"
provider: userPools
operations: [read, create, delete]
}
{ allow: private, provider: userPools, operations: [read] }
]
) {
type: String! # always set to 'post'. used in the SortByTimestamp GSI
id: ID!
content: String!
owner: String
timestamp: Int!
}
(I tried adding and removing the exclamation to the ID here)
Screenshot