It won't work as the link is an "inline element" and text:align only works on "block level elements". This however does:
Either
<p style="text-align:center;"><a href="index.html" >Click here</a></p>
or better still:
in your document head or external stylesheet:
<STYLE TYPE="text/css">
<!--
.centeralign {text-align:center}
-->
</STYLE>
Then in the HTML
<p class="centeralign"><a href="index.html" >Click here</a></p>
You're thinking what's the difference. Difference is with the style declared separately you can change it and it will change all over the page (or site if the style's external) whereas doing it inline (in the p tag) you have to change every one.