Try post content in index

assembler/templates/index.html

@@ -1,7 +1,9 @@
 <!-- wp:template-part {"slug":"header"} /-->
 
-<!-- wp:group {"tagName":"main","metadata":{"name":"Main"},"style":{"spacing":{"blockGap":"0","margin":{"top":"0"}}}} -->
-    <main class="wp-block-group" style="margin-top:0"></main>
+<!-- wp:group {"tagName":"main","metadata":{"name":"Main"},"style":{"spacing":{"blockGap":"0","margin":{"top":"0"}}},"layout":{"type":"default"}} -->
+<main class="wp-block-group" style="margin-top:0">
+    <!-- wp:post-content {"lock":{"move":false,"remove":false},"layout":{"type":"constrained"}} /-->
+</main>
 <!-- /wp:group -->
 
 <!-- wp:template-part {"slug":"footer"} /-->