Have a good read through the PHP strings documentation (at least the "Single quoted" section).
Dealing with your issue in particular, note the following taken from that page
To specify a literal single quote, escape it with a backslash (\).
With that, your code would become:
echo '<td><form action="salesdelete.php" method="POST"><input type="submit" name="submit" value="Delete" onClick="confirm(\'Complete Sale?\')"><input type="hidden" name="invoice_no" value='.$row['invoice_no'].'></form>';