Installing Java on the host does not change the runtime inside a Pterodactyl container. For Unsupported Java Version or UnsupportedClassVersionError, check the selected image against the server software’s requirements.
Verify the active Java version
Look for the Java version in the startup output. If you can access the container shell, run this command there. Running it on the host can report an unrelated installation.
java -version
PaperMC lists Java 25 for Paper 26.1 and newer. Check the appropriate row for older releases. Select the matching Java image in the panel and restart. If the choice is missing, ask the panel administrator to check the egg’s allowed images.
If the same error returns, identify the class that failed to load. A plugin class calls for checking the plugin’s Java and Minecraft compatibility separately.