How to set Approve button not to disappear after I click it

Remove one or both of the conditions in your If statement: 

If(IsNull([Order Approval Date]) and IsNull([Request Rejection Date]), $urlToExecute & "&rdr=" & URLEncode($RefreshPage) )

Your button is dropping because either of the order approval date or rejection date is being populated, I assume with the button. The above states that both have to be null for the button to show. You could just do: 

$urlToExecute & "&rdr=" & URLEncode($RefreshPage)

By itself if you wanted the button there no matter what .