How to create sub-taxonomies ?

Hello everybody,
In my new project, I should have some taxonomies like below.
Taxonomies map:

Main taxonomy( hierarchical )
|
|———-> sub taxonomy 1 ( hierarchical )
|
|———-> sub taxonomy 2 ( hierarchical )
|
|———-> sub taxonomy 3 ( hierarchical )
|
|———-> sub taxonomy 4 ( hierarchical )

I create Main taxonomy:
https://www.example.com/main-taxonomy

but can’t connect sub taxonomies to main taxonomy like this:
https://www.example.com/main-taxonomy/sub-taxonomy-1

I’m really confused and from 2 days I’m fighting with the sub taxonomies.

How to create sub-taxonomies ?

Any idea?