I think this is because the link inside a table is a reference to javascript call and not a page. So following_link that uses curl don't know what to do with that.
From the web page, this is a <a> you are selecting
<a id="ContentPlaceHolder1_UCNested_grdRTAList_RTAIDHyperLink_0" href="javascript:__doPostBack('ctl00$ContentPlaceHolder1$UCNested$grdRTAList$ctl02$RTAIDHyperLink','')">Comprehensive and Progressive Agreement for Trans-Pacific Partnership (CPTPP)</a>
You see that href is not a url but a javascript call.
I think you need to use other solution that can deal with that.
rvest is not enough here I think