Vue.js Error: Unknown custom element <el-submenu> - How to fix it

This error message is related to Vue.js, a popular JavaScript framework for building user interfaces.

The error occurs when Vue.js encounters a component (in this case, '') that it does not recognize or cannot find. This can happen if the component is not registered correctly in the parent component or in the Vue.js application.

To fix this error, you need to make sure that the component is registered correctly. This can be done by importing the component and registering it in the parent component or in the Vue.js application.

If the component is a recursive component, you also need to provide the 'name' option to avoid this error. This is because recursive components can cause an infinite loop, and providing a name helps Vue.js keep track of the component hierarchy and prevent this from happening.