On Windows, SQL*Plus relies on ORACLE_HOME in the registry and the PATH environment variable to locate its message files. Conflicts from multiple Oracle homes (e.g., a client install and a database install) often cause this error.
If the basic troubleshooting steps don't resolve the issue, you may need to perform advanced troubleshooting steps:
Error 57 initializing SQL*Plus Error loading message
This error can occur due to a variety of reasons, including: