insert reqUser is throwing error:User not inserted.How to populate it from below code.
Pls help.

Vf page:

<apex:page standardController="Request__c" extensions="LocationRequestController" sidebar="false">
<apex:form id="theform">
    <apex:pagemessages id="errorMessage"/>
    <apex:pageBlock >
    <apex:pageBlockButtons location="top">
        <apex:commandButton status="statusId" value="Save" action="{!Save}" />
        </apex:pageBlockButtons>
        <apex:pageBlockSection id="atts">
      <apex:pageBlockSectionItem >
            <apex:outputLabel for="ReqUser" value="User"/>
              <apex:selectList id="ReqUser" value="{!usersOption}" size="1">
                <apex:selectOptions value="{!Users}"/>
              </apex:selectList>     
            </apex:pageBlockSectionItem>

</apex:pageBlockSection>
         </apex:pageblock>
      
   </apex:form>   
</apex:page>



Controller:


public with sharing class LocationRequestController {
public Request__c ReqUser{get; set;}
public Order OrderReq{get; set;}
private string AccId{get; set;}

 
    public LocationRequestController(ApexPages.StandardController controller) {
       
         AccId= ApexPages.currentPage().getParameters().get('AccId');
         system.debug('AccId'+ AccId);
         ReqUser=(Request__c) controller.getRecord();
         ReqUser.Location__c=AccId;
         //OrderReq=(Order) controller.getRecord();
       
    }
   
   
  
     public string usersOption{get;set;}
     public List<SelectOption> getUsers() {
                list<SelectOption> optionList = new list<SelectOption>();
                List<Contact> usrList = [Select Id,Name From Contact where AccountId=:AccId];
                for(Contact xUsr: usrList)
                {
                        optionList.add(new SelectOption(xUsr.Id,xUsr.Name));
                }
              
                return optionList;
        }
public pageReference Save() {
    
      insert ReqUser;
}