a combo box event changes a button event

i have tried inserting the button_clicked event into combobox_selectedindexchanged event but generates 2 errors 1.) CS1022 Type or namespace definition, or end-of-file expected 2.) this only occur if i insert the click event into the other event and the error is when i change the button's text into "ADD" but without the click event it is fine and working so any ideas on how to do this?

private void cmbOperation_SelectedIndexChanged(object sender, EventArgs e)
    {
        string selected = (string)cmbOperation.SelectedItem;

        if (selected == "ADD")
        {
            txtID.ReadOnly = false;
            txtLName.ReadOnly = false;
            txtFName.ReadOnly = false;
            txtMI.ReadOnly = false;
            txtGender.ReadOnly = false;
            txtAge.ReadOnly = false;
            txtContact.ReadOnly = false;

            btnOperate.Text = "ADD CLIENT";
        }
    }