On Configuration > Discovery > Create Discovery Rule:

Attempting to add any check with a field (such as port field, community name, etc) is not possible as any such check generates an error message as show in the attached screenshot. ICMP checks which do not involve a settings field CAN be added but any other check with a text field cannot.

Web developer tools show the server is returning the following to the check validation call to discoveryconf.php:

<div class="msg-bad">Zabbix has received an incorrect request.<div class="msg-details"><span class="link-action" onclick="javascript: showHide($(this).next('.msg-details-border'));">Details</span><ul class="msg-details-border"><li>Operation cannot be performed due to unauthorized request.</li><li>Operation cannot be performed due to unauthorized request.</li><li>Operation cannot be performed due to unauthorized request.</li></ul></div><span class="overlay-close-btn" onclick="javascript: $(this).closest('.msg-bad').remove();" title="Close"></span></div>