I  have simple Sobject in my bd, with 2 number fields: index1__c, index2__c:

on my visualforce page i'm going to create record of this object only when these filelds are equal.

if they aren't I want to render another field of this objcet which says why they aren't equal

Creating the record must be obtained by pressing enter key. 

On pressing enter nothing happens.

Is any restrictons where  <apex:actionSupport> tag must be placed in? 

VF page 

<apex:page id="HM6" controller="HM6Controller">
    <apex:form >
        <apex:pageBlock >
            <apex:pageBlockSection >
                <apex:inputField value="{!hm.index1__c}"/>
                <apex:inputField value="{!hm.index2__c}"/>
               
             
            </apex:pageBlockSection>
            
        </apex:pageBlock>
        
        <apex:outputpanel id="block">
        
         <apex:outputField value="{!hm.reason_not_equals__c}" rendered="{!NOT(check)}" />
          <apex:actionSupport event="onkeypress" action="{!save}" reRender="block" />
          
        </apex:outputpanel>/>
        <apex:pageMessages />
    </apex:form>
</apex:page>

controller

/**
 * Created by LanaPC on 22.06.2016.
 */

public with sharing class HM6Controller {
    public HM6__c hm;

    public HM6__c getHM(){
        return hm;

    }
    public boolean check;

    private String fillMessalge(Decimal index1, Decimal index2){
        if(index1>index2) {
            return 'index1>index2';
        }else return 'inde1<index2';
    }

    public HM6Controller(){
        hm=new HM6__C();

    }

    public Boolean getcheck(){
        if (hm.index1__c==hm.index2__c){
        check = true;
        return check;
        }
        else {
           hm.reason_not_equals__c= fillMessalge(hm.index1__c,hm.index2__c);
            check= false;
            return  check;
        }
    }

    public PageReference save(){
        if (getcheck()) insert hm;
        return null;
    }


}