You have to buy a product that connects your mixer to your computer. Rekordbox makes one, but there are other companies that make them too.

https://www.amazon.com/Pioneer-INTERFACE2-2-Channel-Audio-Interface/dp/B0756LVTY6/ref=sr\_1\_8?crid=28U3R44CC7C6A&keywords=rekordbox&qid=1651981665&sprefix=rekordbox%2Caps%2C145&sr=8-8