Because it takes 3 business days for the actual transfer of stock to occur after you buy or sell to the next owner, your cash is tied up until that happens. This is called the settlement period.
Therefore, brokers offer "margin", which is a form of credit, or loan, to allow you to keep trading while the settlement period occurs, and in other situations unrelated to the presented question. To do this you need a "margin account", you currently have a "cash account".