This is a known issue in the beta version. Embeds should be working fine in the current version. You can track progress on embeds in the beta version on this development ticket. https://github.com/LibraryOfCongress/viewshare/issues/96
Admintrow (Admin, Viewshare) commented
Is this on the beta site or the normal site? Also, what browser are you using? I just tried this in chrome and it worked OK, so it might be a browser issue. It may also have just been a server issue that has since been resolved.