Now code coverage is 74%, Please check updated Class and Test class

@RestResource(urlMapping='/api/IVR/*')
global class IVRRestAPI 
{
  
    @HttpPost
    global static List<IVR__c> CreateIVR(List<createIVR> icretIVR) 
    {
        try{
        Set<String> loanId = new Set<String>();
        List<Loan__c> loanlst = new List<Loan__c>();
        Map<String,String> mpidtoname = new Map<String,String>();
        List<IVR__c> IVRlst = new List<IVR__c>();
        for(createIVR icret:icretIVR)
        {
            if(icret.LoanID != null)
            {
                loanId.add(icret.LoanID);
            }
        }
        System.debug('loanId '+loanId);
        if(loanId.size() > 0)
        {
            loanlst = [SELECT Id,Name FROM Loan__c WHERE Name IN:loanId];
        }
        System.debug('loanlst '+loanlst);
        if(loanlst.size() > 0)
        {
            for(Loan__c iloan:loanlst)
            {
                mpidtoname.put(iloan.Name,iloan.Id);
            }
        }
        System.debug('mpidtoname '+mpidtoname);
        for(createIVR icret:icretIVR)
        {
            IVR__c IVR = new IVR__c();
                        
            if(mpidtoname.containskey(icret.LoanID))
            {
                IVR.Loan_Id__c = mpidtoname.get(icret.LoanID);
            }
            
            IVR.Name = icret.IVRId;
            IVR.Merchant_Name__c = icret.MerchantName; 
            IVR.IVR_Number__c = icret.IVRNumber;
            IVR.Sanction_Amount__c = Decimal.valueOf(icret.SanctionAmount);
            IVR.Disbursed_Amount__c = Decimal.valueOf(icret.DisbursedAmount);
            IVRlst.add(IVR);
        }
        if(IVRlst.size() > 0)
        {
            insert IVRlst;
            return IVRlst;
        }
        return null;
        }catch(exception e){System.debug(e.getmessage() + e.getlinenumber());return null;}
    }
    global class createIVR
    {
        public String IVRId;
        public String LoanID;
        public String DisbursedAmount;
        public String SanctionAmount;
        public String MerchantName;
        //Public String DisbursedDateTime;
        Public String IVRNumber;
        
        public createIVR(String IVRId,String LoanID,String DisbursedAmount,String MerchantName,String SanctionAmount,String IVRNumber){
            this.IVRId = IVRId;
            this.LoanID = LoanID;
            this.DisbursedAmount = DisbursedAmount;
            this.MerchantName = MerchantName;
            this.SanctionAmount= SanctionAmount;
            this.IVRNumber = IVRNumber;       
        }        
    }
}

@istest
public class IVRRestAPITest{
 static testMethod void  IVRRestAPITest(){

   IVRRestAPI reqst=new IVRRestAPI();
   String JsonMsg=JSON.serialize(reqst);

   RestRequest req = new RestRequest();  
    req.addHeader('Content-Type', 'application/json'); // Add a JSON Header as it is validated
    req.requestURI = '/services/apexrest/api/IVR/CreateIVR';  //Request URL
    req.httpMethod = 'POST';//HTTP Request Type
    req.requestBody = Blob.valueof(JsonMsg);
    
    RestResponse res = new RestResponse();
    RestContext.request = req;
    RestContext.response= res;

    Test.startTest();
    List<IVRRestAPI.createIVR> icretIVRList  = new List<IVRRestAPI.createIVR>();
    Loan__c loan = new Loan__c();
    loan.name='Test';
    insert loan;
    icretIVRList.add(new IVRRestAPI.CreateIVR('Name','Loan.ID','DisbursedAmount','SanctionAmount','MerchantName','IVRNumber') );
    IVRRestAPI.CreateIVR(icretIVRList);
    Test.stopTest();
   }
}