It usually means that the jar file set in Multicraft is not found in the correct location.
How to fix this error:
Check your custom jar settings.
- If you are trying to use a custom jar, make sure the jar is named correctly. Log into your FTP panel and check the name of the jar, which should be set to "my.jar"
- If you are not trying to use a custom jar, make sure you have the option disabled. To disable the feature, navigate to multicraft where there's a dropdown in the middle of the page to select the correct jar