How does the pecifying button type — cmd _xclick-subscriptions subscribe button work?

[Shawnchen11]

I am using the following code to use the auto subscribe feture

<form class="form_box" action="https://www.paypal.com/cgi-bin/webscr" method="POST" target="_blank">
<input type="hidden" name="cmd" value="_xclick-subscriptions"/>
<input type="hidden" name="business" value="business email"/>
<input type="hidden" name="item_name" value="item_name"/>
<input type="hidden" name="item_number" value="123456"/>
<input type="hidden" name="a3" value="9.90"/>
<input type="hidden" name="p3" value="1"/>
<input type="hidden" name="t3" value="M"/>
<input type="hidden" name="src" value="1"/>
<input type="hidden" name="no_note" value="1"/>
<input type="hidden" name="modify" value="1"/>
<input type="hidden" name="currency_code" value="USD"/>
<input type="hidden" name="notify_url" value="notify_url"/>
<input type="hidden" name="custom" value="custom value" />

<input type="button" value="Subscribe"/>
</form>

However, I get the following error

1. You must add funds to your PayPal account before sending more money.