`plugin is enabled but not installed` error when Kong can't locate the plugin's source code

See “xxx plugin is enabled but not installed” like below in the Kong error log:

no plugin found; on plugin 'response-transformer-advanced': response-transformer-advanced plugin is enabled but not installed;
no plugin found
stack traceback:
	[C]: in function 'assert'
	/usr/local/share/lua/5.1/kong/init.lua:553: in function 'init'
	init_by_lua:3: in main chunk

This error means Kong could not find the source code of the xxx plugin, where xxx is the actual plugin name. The plugin code is usually located in the /usr/local/share/lua/5.1/kong/plugins/<plugin-name> directory. Check whether the xxx plugin’s code is present in the /usr/local/share/lua/5.1/kong/plugins/xxx directory.

There are three common causes and fixes:

1. You are using the wrong Kong image. Some plugins are only supported by Enterprise Kong images and can’t be used with OSS Kong images. Use an Enterprise Kong image in this case.
2. The /usr/local/share/lua/5.1/kong/plugins/xxx directory has been deleted. Add the /usr/local/share/lua/5.1/kong/plugins/xxx directory back and restart Kong.
3. You mounted the /usr/local/share/lua/5.1/kong/plugins/xxx directory to an external volume. Stop mounting the directory and restart Kong.