It appears there is some technical issue with your subscription button.
Add one sample code for your reference or you could refer to the integration guide here.
<form action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_xclick-subscriptions">
<input type="hidden" name="business" value="[email protected]">
<input type="hidden" name="item_name" value="TMC membership fee">
<input type="hidden" name="item_number" value="1001">
<input type="hidden" name="no_note" value="1">
<input type="hidden" name="a1" value="1.00">
<input type="hidden" name="p1" value="1">
<input type="hidden" name="t1" value="M">
<input type="hidden" name="a2" value="2.00">
<input type="hidden" name="p2" value="1">
<input type="hidden" name="t2" value="M">
<input type="hidden" name="a3" value="3.00">
<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="currency_code" value="AUD">
<input type="hidden" name="bn" value="PP-SubscriptionsBF:btn_subscribeCC_LG.gif:NonHostedGuest">
<input type="image" src="https://www.paypalobjects.com/en_US/GB/i/btn/btn_subscribeCC_LG.gif" border="0" name="submit" alt="PayPal — The safer, easier way to pay online.">
<img alt="" border="0" src="https://www.paypalobjects.com/en_GB/i/scr/pixel.gif" width="1" height="1">
</form>