A sole trader is a self-employed person who carries out a business. An employed person has a paid job.
A person can register as a sole trader whilst being employed and can therefore be both employed and self-employed at the same time. For example, a person can work for their employer Monday to Friday but work for their own business on the weekends.
You might want to check the terms of your employment agreement before setting up another business but if there is no specific clause to this effect than you can register as a sole trader.