When you are using a postback pixel, affiliates must use postbacks as well.
Enabling this across the board in AFFILIATE PORTAL SETTINGS
You can enable this feature across the board by going to SET UP > AFFILIATE PORTAL SETTINGS.
The first setting under 'Affiliate Permissions' is to 'Only Allow Affiliate Postbacks'.
With this setting enabled, affiliates will no longer be able to place any pixel other than a postback through the Affiliate Portal as seen below.
With this global setting enabled, every campaign will now have a warning icon with the below message stating that this setting has been enabled.
NOTE: If you have a postback placed with your Advertiser, placing an HTML pixel even in the admin will not function.