There's nothing wrong with your query--I just don't think the W3Schools online database tool supports the EXCEPT clause. You would need to download a database that supports the EXCEPT clause if you wanted to practice it (SQL Server, Oracle DB, PostgreSQL).

Try this equivalent (and more performant) query

select orderid

from orders

where customerid >= 100

The EXCEPT clause runs but there are not customerid greater or equal than 100.