In addition to the XRExtras library from 8th Wall you need to add the A-FRAME extras library. You will need to use the AFRAME extras to use the animation-mixer component
You can check out how to add the animation-mixer component in line 10 here: A-Frame: Animation Mixer | 8th Wall | 8th Wall
Make sure to add the library in your head.html :
<!-- Other external scripts and meta tags can also be added. --> <script src="//cdn.8thwall.com/web/aframe/aframe-extras-6.1.1.min.js"></script> <!-- The above library includes the animation-mixer component -->