Why does preflight fail?
Why does preflight fail? A preflight could be successful, but the request could still fail for many reasons, such as a file not found, an authorization error, or a server issue. The preflight only ensures that the browser can make a cross-origin request to the server, and nothing more.
How do I turn on preflight?
You can also access the Preflight panel from the top menu, under Window > Output > Preflight. First up, at the top left of the panel you can see a check box for turning Preflight On and Off. As a rule, you should always have Preflight set to On.