:toc: left
is not a valid option for embedded asciidoc. See [1] for details.
However, there is still hope, because you can change your stylesheet.
First, change your templates in order to add the classes "toc2 toc-left"
to the body tag. This makes room for the toc on the left side.
Then copy the toc2
-styles from the original aciidoctor stylesheet to the toc
-style by copy & psting this into the header:
<style>
@media only screen and (min-width:768px){
#toctitle{font-size:1.375em}
#toc.toc{margin-top:0!important;background-color:#f8f8f7;position:fixed;width:15em;left:0;top:0;border-right:1px solid #efefed;border-top-width:0!important;border-bottom-width:0!important;z-index:1000;padding:1.25em 1em;height:100%;overflow:auto; padding-top: 60px;}
#toc.toc #toctitle{margin-top:0;margin-bottom:.8rem;font-size:1.2em}
#toc.toc>ul{font-size:.9em;margin-bottom:0}
#toc.toc ul ul{margin-left:0;padding-left:1em}
#toc.toc ul.sectlevel0 ul.sectlevel1{padding-left:0;margin-top:.5em;margin-bottom:.5em}
}
@media only screen and (min-width:1280px){
body.toc2{padding-left:20em;padding-right:0}
#toc.toc{width:20em; padding-top: 60px;}
#toc.toc #toctitle{font-size:1.375em}
#toc.toc>ul{font-size:.95em}
#toc.toc ul ul{padding-left:1.25em}
}
</style>
this should solve your problem.
[1] https://asciidoctor.org/docs/user-manual/#using-a-toc-with-embeddable-html