You cannot use a Property as the value in the data table on line 13 of your visualforce page. I have created 2 controller methods called getpos1 and getpos2. 

public class select_rec_condition_class {
    List<Position__c> pos1{get;set;}
    
    public List<Position__c> getpos1(){
        return pos1;
    }
    public void LessSalary()
    {
        pos1 = [select Name,Location__c,Min_Pay__c,Max_Pay__c,Open_Date__c,Close_Date__c,
                Status__c from Position__c where Min_Pay__c <= 2000];
    }
    List<Position__c> pos2{get;set;}
    public List<Position__c> getpos2(){
        return pos2;
    }
    public void MoreSalary()
    {
        pos2 = [select Name,Location__c,Min_Pay__c,Max_Pay__c,Open_Date__c,Close_Date__c,
                Status__c from Position__c where Min_Pay__c > 2000];   
    }
}

Also fix the typos in your visualforce page, on lines 19 and 32. It should be Close_Date__c not Cloe_Date__c. 
Below is the visualforce page, only change is the corrected typos.

<apex:page showHeader="false" controller="select_rec_condition_class">
 <apex:form>
  <apex:pageBlock>
   <center>
   <apex:commandButton value="LowSalary" action="{!LessSalary}"/>&nbsp;&nbsp;
   <apex:commandButton value="HighSalary" action="{!MoreSalary}"/>
   </center>
      <font color="red" size="4"><br/>
   Position records whose minpay is less than or equal to 2000
  </font><br/>
  <apex:pageBlockTable value="{!pos1}" var="ps1">
    <apex:column value="{!ps1.Name}"/>
    <apex:column value="{!ps1.Location__c}"/>
    <apex:column value="{!ps1.Min_Pay__c}"/>
    <apex:column value="{!ps1.Max_Pay__c}"/>
    <apex:column value="{!ps1.Open_Date__c}"/>
    <apex:column value="{!ps1.Close_Date__c}"/>
    <apex:column value="{!ps1.Status__c}"/>
  </apex:pageBlockTable>
      <br/><br/>
   <font color="blue" size="4">
   Position records whose minpay is greater than 2000
  </font><br/>
  <apex:pageBlockTable value="{!pos2}" var="ps2">
    <apex:column value="{!ps2.Name}"/>
    <apex:column value="{!ps2.Location__c}"/>
    <apex:column value="{!ps2.Min_Pay__c}"/>
    <apex:column value="{!ps2.Max_Pay__c}"/>
    <apex:column value="{!ps2.Open_Date__c}"/>
    <apex:column value="{!ps2.Close_Date__c}"/>
    <apex:column value="{!ps2.Status__c}"/>
  </apex:pageBlockTable>
  </apex:pageBlock>
 </apex:form>
</apex:page>