In the WordPress admin, you can align test in the center using one of the WYSIWYG buttons. Maybe try that…
If not, you can also try adding this to your child theme or custom CSS:
.iframe {
margin: 0 auto;
text-align: center;
}
Building off of Christines code:
.iframe {
display: block;
margin: 0 auto;
text-align: center;
}
Display block is required to get things centered in the container.