Hello,

Modify the below sample code :

VF Page :
----------------------------------------------------
<apex:page controller="sampleCon">

<script type="text/javascript">
     
    function getActive(selectedValue) 
    {  
    if( selectedValue == 'YES')
    {
    var ele=document.getElementById('{!$Component.form1.dropdown}');
      ele.disabled =false;
    }else if(selectedValue == 'NO')
    {
         var ele=document.getElementById('{!$Component.form1.dropdown}');
      ele.disabled =true;
    }
            
    } 
    </script>
    <apex:form id="form1">
        <apex:selectRadio value="{!selectedValue}" layout="pageDirection" onchange="getActive(this.value);">
            <apex:selectOption itemValue="YES" itemLabel="YES"/>
            <apex:selectOption itemValue="NO" itemLabel="No"/>
            </apex:selectRadio><p/>
            
            <apex:selectList id="dropdown" size="1">
               <apex:selectOption itemValue="YES" itemLabel="YES"/>
            <apex:selectOption itemValue="NO" itemLabel="No"/>
            </apex:selectList>
            
  </apex:form>
</apex:page>
---------------------------------------------

Controller :
-------------------------
public class sampleCon {

public String selectedValue{get;set;}
}
------------------------------

Let us know if this helps.

Thanks,
Shravan