paper-plugin.yml: "[api-version] ... A value is required for this field"

Server log

[DirectoryProviderSource] Error loading plugin: [api-version] of type org.bukkit.craftbukkit.util.ApiVersion: A value is required for this field

What it means

In paper-plugin.yml, api-version is required. Without it Paper refuses the plugin.

What causes it

paper-plugin.yml has no api-version.

How to fix it

Before

name: MyPlugin
version: '1.0'
main: com.example.myplugin.MyPlugin

After

name: MyPlugin
version: '1.0'
main: com.example.myplugin.MyPlugin
api-version: '26.3'

Also note that paper-plugin.yml ignores commands, depend, softdepend and libraries without a warning. Register commands in code and declare dependencies under dependencies: server:.

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