When I try to drag an Element this error shows up only in Safari.

Have you included the Web Animations polyfill in the site? Safari does not support Web Animations yet, so it needs the polyfill. It's also mentioned in Muuri's readme.
Closing this since I cannot reproduce this and no reduced test case is provided.
Most helpful comment
Have you included the Web Animations polyfill in the site? Safari does not support Web Animations yet, so it needs the polyfill. It's also mentioned in Muuri's readme.