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While most mods and plugins are standalone installations that only require you to upload the respective file to the mods or plugin folder, some require additional dependencies in order to run. These dependencies are referred to as core files because without them, the respective mod or plugin would not work. Generally when your server crashes as a result of this error you are given a detailed line telling you what mod/plugin is failing to load and what mod/plugin you need in order to make it load.
To fix this error you can either remove the mod causing the issue or install the missing dependency for that mod or plugin. To know which mod or plugin you need to add or remove, review your server console.