There are three primary historical and technical reasons for this:

Have you successfully sideloaded using a modern tool? Share your experience in the comments below, and stop pulling your hair out over obsolete assertion errors.

This article will provide a deep dive into what the "assert code 200" error means, why it happens, how to potentially fix it, and—most importantly—what modern tools have replaced Cydia Impactor. To understand the error, you must understand the tool. Cydia Impactor was unique because it used developer authentication to sign apps. Normally, to install an app, you need a paid Apple Developer account ($99/year) or a free personal team account. Cydia Impactor bridged this gap by allowing users to sign apps using their standard Apple ID credentials. The tool would generate a digital certificate, inject it into an .ipa file, and install it directly onto a connected iOS device.

In layman’s terms: Cydia Impactor knocked on Apple’s door, Apple opened it, but instead of handing Impactor the "app signing keys," Apple handed it a piece of paper that said, "Your request method is invalid." The assert code 200 error is Cydia Impactor’s way of saying: "I tried to use the old, deprecated API to get a signing certificate, but Apple has shut down that API or changed the rules, and I don't know how to handle this response."