Hi there,
I have been working on some integration class, and i am new to integration things.
Below is my main class that reads the payload data and updates it, under that i try to created Test class for it but  when i Run that test class it gives me an error like in .img.
There is also a Flow which invokes when Account record gets updated.
Main Class :

@RestResource(urlMapping='/accountInterestUpdate/*')
global with sharing class accountInterestUpdate {
    @HttpPost
    global static void doPost() {
        Map<String, Object> requestBody= (Map<String, Object>)JSON.deserializeUntyped(RestContext.request.requestBody.toString());
        Map<String, Object> requestBody1 = new Map<String, Object>();
        for(String s: requestBody.keySet()){
            requestBody1.put(s.toLowerCase(),requestBody.get(s));
        }
        system.debug('requestBody' +requestBody);
        String buyerId = String.valueOf(requestBody1.get('buyerid'));
        String subStatus = String.valueOf(requestBody1.get('interested'));
        String buyerEmpId = String.valueOf(requestBody1.get('buyeremployeeid'));
        String subsId = String.valueOf(requestBody1.get('subscriptionid'));
        Account account = [SELECT ID, Name, API_Buyer_Id__c, Contact_Buyer_Id__c, Interested__c FROM Account WHERE API_Buyer_Id__c = :buyerId LIMIT 1];
        if(subStatus == 'true'){
            account.Interested__c = true;
            account.Contact_Buyer_Id__c = buyerEmpId;
        }
        else{
            account.Interested__c = false;
            account.Contact_Buyer_Id__c = buyerEmpId;
        }
       update account;
    }
}

Test Class :

@isTest
public class accountInterestUpdateTest {
    @isTest 
     static void testPostMethod(){
        Test.startTest();
        Account acc = new Account();
        acc.Name = 'Test Account';
        acc.API_Buyer_Id__c = '234567';
        acc.Interested__c = true;
        insert acc;
        RestRequest req = new RestRequest(); 
        RestResponse res = new RestResponse();
        req.requestURI = '/services/apexrest/accountInterestUpdate/';
        String JSONMsg = '{"buyerId" : "'+acc.API_Buyer_Id__c+'","Interested":"true"}';
        req.requestBody = Blob.valueof(JSONMsg);
        req.httpMethod = 'POST';
        RestContext.request = req;
        RestContext.response= res;
        accountInterestUpdate.doPost();
        Test.stopTest();
    }
     @isTest static void testPostMethodInterestedFalse(){
        Test.startTest();
        Account acc = new Account();
        acc.Name = 'Test Account';
        acc.API_Buyer_Id__c = '234567';
        insert acc;
        Contact con = new Contact();
        con.FirstName = 'test';
        con.LastName = 'contact';
        con.AccountId = acc.id;
        insert con;
        RestRequest req = new RestRequest(); 
        RestResponse res = new RestResponse();
        req.requestURI = '/services/apexrest/accountInterestUpdate/';
        String JSONMsg = '{"buyerId" : "'+acc.API_Buyer_Id__c+'","Interested":"false"}';
        req.requestBody = Blob.valueof(JSONMsg);
        req.httpMethod = 'POST';
        RestContext.request = req;
        RestContext.response= res;
        accountInterestUpdate.doPost();
        Test.stopTest();
    }
}

.img


So, if anyone know anything, please help me in that. Any help is appreciated.