This is relatively simple, but doesn’t really have any user ability to effect. This error occurs when Apple Developer Program website/service is down for maintenance, scheduled or otherwise. The Cydia Impactor tool actually explicitly requires access to the developer portal in order to request the necessary privileges and sign the IPA file before sideloading it onto the device. Apple’s Developer Program service must be functioning for that to happen.
In order to get up and running as quickly as possible, you can follow the status of the developer portal at developer.apple.com/system-status. As soon as it comes back online, you can start the jailbreak process again.