The syntax error is because you need a space between "Property" and "WHERE" - at the moment your select statement evaluates to:
"SELECT * FROM PropertyWHERE Sale_Rent='Rent'"
In addition:
1) Tag your question with ASP.NET, rather than C# - a DropDownList with a Value property does not exist outside ASP.NET.
2) Try to get into the habit of using parameterized queries: I know that your existing code has no risk of an SQL Injection attack, but when you re-use this code in a month for you text box, you won't think about it. It is well worth getting into the habit early, and using it at all times.
3) Why didn't you try using the debugger? That would have shown you the problem in thirty seconds!