Visual Studio C# web form checkbox CheckedChanged method error

this is my fist question, I have been searching about how to use checkbox in a c# web form, and I found some codes that almost is what I need, but I keep getting an error because all those codes does not apply for C# web forms specifically.

File.aspx has this code:

<asp:CheckBox ID="cbxScheduleDate" runat="server" Text="Schedule for today" AutoPostBack="true"; OnCheckedChanged="cbxScheduleDate_CheckedChanged"/>

File.aspx.cs has this code:

protected void cbxScheduleDate_CheckedChanged(object sender, EventArgs e)
    {
        if (cbxScheduleDate.Checked)
            txtScheduleDate.Visible = false;
        else
            txtScheduleDate.Visible = true;

    }

So, what I need is when the checkbox is checked I make the txtScheduleDate textbox not visible.

Right now with that code, Visual Studio has the error:

The name 'cbxScheduleDate' does not exist in the current context

Does anyone now how to solve this Visual Studio error, or another way to make this work?

Thank you!