Must declare the scalar variable "@datetime"

I am getting an error in a Try/Catch Textbox I have set up:

"Must declare the scalar variable "@datetime"."

Here is the code in question:

TextBox1.Text = System.DateTime.Now.ToString()

cmd.Parameters.AddWithValue("@datetime", TextBox1.Text)

There is an auto increment variable before it: idr int identity(1,1) not null.