Search for <b:if cond='data:post.labels'>
and add below line after first </b:loop>
The Line is
<meta name="keywords" content="<b:loop values='data:post.labels' var='label'><data:label.name/><b:if cond='data:label.isLast != "true"'>,</b:if></b:loop>" />
And Save the Template.. You are Done
See the Demo Here
and add below line after first </b:loop>
The Line is
<meta name="keywords" content="<b:loop values='data:post.labels' var='label'><data:label.name/><b:if cond='data:label.isLast != "true"'>,</b:if></b:loop>" />
And Save the Template.. You are Done
See the Demo Here