First, type in a search such as “3D” or “vr180”. In the search results, click on “filters,” and under features, select “VR180” or “3D”. For 3D 360 content, it is better to search for “3D 360” to distinguish them from regular 360 videos.

Try updating your browser or clear your browser's cache & cookies. You can also check whether your browser extensions that block ads are affecting video playback. As another option, try opening YouTube in an incognito window with all extensions disabled and check if the issue continues.