What did Joe the system administrator likely forget to do when he encountered an error joining the new print server to the existing domain?

The most likely reason Joe the system administrator encountered an error when joining the new print server to the existing domain revolves around network settings. Properly configuring network settings is crucial for any server to communicate effectively within a domain. This includes verifying elements such as the server's IP address, subnet mask, default gateway, and DNS settings.

If these configurations are incorrect or incomplete, the server may not be able to locate the domain controller or establish a domain connection, leading to failure in joining the domain. Network connectivity issues are often a common source of errors during domain join processes, making this a critical step for Joe to have verified.

For the other options: recreating the printer on the new server, migrating the print server database, or restoring a backup are important tasks for ensuring print functionality or continuity; however, they do not directly relate to the necessary steps for joining a server to a domain. Thus, the foundational requirement of having the network settings appropriately configured takes priority in this scenario.