Update Option Error: Notice: Undefined index

This is because when a checkbox isn’t checked it didn’t send to the server. This is why the POST array doesn’t have the upo_enable index.

You can, however, check if the value is empty or not:

$upo_enable = !empty($_POST['upo_enable']) ? true : false;

Or

$upo_enable = !empty($_POST['upo_enable']) ? $_POST['upo_enable'] : false;

More on this:
https://www.w3.org/TR/html401/interact/forms.html

checkboxes

Checkboxes (and radio buttons) are on/off switches that may be toggled
by the user. A switch is “on” when the control element’s checked
attribute is set. When a form is submitted, only “on” checkbox
controls can become successful.