instead of
"onfocusin", Please place "onfocus". let see the modified code below.
<asp:TextBox ID="txtAddress" TextMode="MultiLine" onfocus ="SelectAll(this.id);" ondblclick="this.focus();this.select()" runat="server" TabIndex="6" MaxLength="300"
SkinID="textarea-small-style" ValidationGroup="requied"></asp:TextBox>