The truncate command empties the SAMPLE table before you insert data back into it, so you shouldn't need any kind of replace command (if you did need it, in oracle it's called MERGE).
Be sure to have a semicolon after each statement.
Put the cursor on the line with the TRUNCATE command and then hit F9. You should see "table truncated" at the bottom of the Editor window as feedback.
Next, put the cursor on the line with the INSERT command and then F9 again. You should see # of rows insert at the bottom of the Editor window as feefback.
Finally, run a COMMIT; command (or click the COMMIT toolbar button) to make sure the data stays there after you disconnect.