The code is working perfectly. The code
ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "clientscript", "alert('Your details have successfully registered with us. Kindly check your mail inbox to login.')", true);
actually registers the javascript and does not fire it. You need to find other way to display the alert from client side. I can not suggest you more on this since you have provided a very little information on this.
Nextly, since you are redirecting the page using
Response.Redirect("Opp_As_DSA.aspx");
, the alert will be displayed and not visible to you because of the redirection.