if you import openapi json to AWS API do you have any errors? Various validation tools may show different messages on the same json. I see only one issue with your json, that type is defined as "x-nullable": false so it cannot be null and must have value, but it does not have "required": true set. So setting IsRequired=true in ApiMember attribute should resolve this validation error in swagger UI, but anyway if you target goal to import to AWS API you show test your json on Amazon API Gateway, because their validation can differ from swagger editor validation.
[ApiMember(Name= "WorkWeek", DataType = "string", Description = "The type of workweek to initialize the schedule with.", ParameterType = "body", IsRequired = true)]