We found the same issue on our site. Installed plugin, but no settings visible. Only “General”, “Send Test Email” etc tabs were visible, but none of them worked.
The theme framework (Bones) has a function that removes a lot of “unnecessary” WordPress features like RSS links, parent post links, etc.
The one causing the issue was something at removes the default version number for loaded scripts. /js/script.js?ver=5.3.3 becomes /js/script.js. When removed the remover, Postmark started working. So it seems that postmarkapp requires the version number on the generated WordPress scripts.
These are the lines we Removed from our theme
// remove WP version from css
add_filter( 'style_loader_src', 'bones_remove_wp_ver_css_js', 9999 );
// remove Wp version from scripts
add_filter( 'script_loader_src', 'bones_remove_wp_ver_css_js', 9999 );
And the function itself
// remove WP version from scripts
function bones_remove_wp_ver_css_js( $src ) {
if ( strpos( $src, 'ver=' ) )
//$src = remove_query_arg( 'ver', $src );
return $src;
}