Centralising the button on a page

You will need to provide a URL to your site to provide feedback.

Since you didn’t specify, I assume you mean the one at the bottom of the homepage ?
You can use this :
Customizer –> Additional CSS :

.wp-block-buttons {
    text-align: center;
}

Or, you can style just that one button by adding :
<div class="wp-block-buttons" style="text-align: center;">

See :

View post on imgur.com