Web form error has an error on Default.aspx

Hi Team

If you have an error when creating a web form, error "Compiler Error Message: CS1061: 'default_aspx' does not contain a definition for 'btnStart_Click' and no accessible extension method 'btnStart_Click' accepting a first argument of type 'default_aspx' could be found (are you missing a using directive or an assembly reference?)

//Default.aspx
"btnStart" runat="server" OnClick="btnStart_Click" /> // Error is on this line, how to resolve this?
"SomeText" runat="server">

//Default.cs
string results;
private async void btnStart_Click(object sender, EventArgs e)
{
SaySomething();
SomeText.Text = results;
}
async System.Threading.Tasks.Task<string> SaySomething()
{
await System.Threading.Tasks.Task.Delay(1000);
results = "Hello result";
return results;
}