Why can I receive bitcoin instantly but can’t send it right away after buying?

 I bought some bitcoin on an exchange using my debit card. The bitcoin showed up in my account immediately. But when I try to send it to my own wallet, it says I have to wait 7 days. Why is that? If I already own the bitcoin, why can't I move it right away? Is this normal or did I pick a bad exchange? Also I keep getting private messages from people offering to help me send it faster. I'm not replying to those because it feels suspicious. Thanks.