Can’t embed any forms using a block editor in Wordpress
I will take a wild guess here since we don't know the versions or Wordpress and CiviCRM and which block. Since you mentioned the block editor, which does not provide a CiviCRM block, I will guess that you are using a TinyMCE block that still displays a CiviCRM button -- perhaps the Classic block or Classic Paragraph block?
In my case (Wordpress 5.4.2 and CiviCRM 5.27.0), I no longer see a CiviCRM button. This is consistent with documentation at https://docs.civicrm.org/sysadmin/en/latest/integration/wordpress/. Instead, I use the Shortcode block or I directly enter a CiviCRM shortcode in a paragraph block, using the shortcode formats described in the doc.