The only purpose of this data type is to represent cases where value is not expected. So you can't create a Nothing type value.

For example, literal NULL has type of Nullable(Nothing). See more about Nullable.

The Nothing type can also used to denote empty arrays:

:) SELECT toTypeName(array())

SELECT toTypeName([])

│ Array(Nothing)      │

1 rows in set. Elapsed: 0.062 sec.