salesforce classic onclick javascript button code :
 

{!REQUIRESCRIPT("/soap/ajax/34.0/connection.js")} {!REQUIRESCRIPT("/soap/ajax/34.0/apex.js")} 
var connection = sforce.connection; 
var qid="{!Quote__c.Id}"; 
var isedit="{!Quote__c.isEdited__c}"; 
var isPreviewed="{!Quote__c.isPreviewd__c}"; 
var isGenerated="{!Quote__c.Generated__c}"; 
var pot="{!Quote__c.potential__c}"; 
var x=pot.replace(/\,/g,''); 

var quot= new sforce.SObject("Quote__c"); 
quot.id=qid; 
if(isGenerated==true && isPreviewed==false){ 
alert('Please preview the quote then generate '); 
} 
else if(isedit==true && isPreviewed==false){ 
alert('Please preview the quote then generate '); 
}else{ 
parent.location.href = parent.location.href; 
window.open("/apex/QuotePDF?id={!Quote__c.Id}","_blank"); 
if(x>50000){ 
if (('{!$User.ProfileId}'=='00e90000000emPJ') || ('{!$User.ProfileId}'=='00e90000001k992' )){ 
alert('Please print the quote for manager signature'); 
}} 
}

visual page button code :
 

<apex:page standardController="Quote__c">
 <apex:includeScript value="../../soap/ajax/34.0/connection.js"/>
  <apex:includeScript value="../../soap/ajax/34.0/apex.js"/>
<script>
sforce.connection.sessionId='{!GETSESSIONID()}';

var qid="{!Quote__c.Id}"; 
var isedit="{!Quote__c.isEdited__c}"; 
var isPreviewed="{!Quote__c.isPreviewd__c}"; 
var isGenerated="{!Quote__c.Generated__c}"; 
var pot="{!Quote__c.potential__c}"; 
var x=pot.replace(/\,/g,''); 

var quot= new sforce.SObject("Quote__c"); 
quot.id=qid; 
if(isGenerated==true && isPreviewed==false){ 
alert('Please preview the quote then generate '); 
} 
else if(isedit==true && isPreviewed==false){ 
alert('Please preview the quote then generate '); 
}else{ 
parent.location.href = parent.location.href; 
window.open("/apex/QuotePDF?id={!Quote__c.Id}","_blank"); 
if(x>50000){ 
if (('{!$User.ProfileId}'=='00e90000000emPJ') || ('{!$User.ProfileId}'=='00e90000001k992' )){ 
alert('Please print the quote for manager signature'); 
}} 
}
</script>
</apex:page>

I can able to show this button in lightening in salesforce .
But I cant see any alert messages ,when i click on the button .it is blank pop up opening.