Exception on trying to install Craft CMS 3 plugin
Please remove the first \ and include a vendor directory

11 namespace [package]\[websitename];
12 
13 use [package]\[websitename]\variables\[websitename]Variable;


The namespace must always consist the package and the plugin name docs

Here is an example

namespace ns\prefix; // &lt;-- see: ns\prefix, not only prefix

class Plugin extends \craft\base\Plugin