Is my bike chain skipping?
Judging from the sound and the video, your chain is getting stuck in the rear derailleur pulleys. Sometimes it’s loose and not engaging on the pulley, and sometimes the chain is routed  over the metal bit between pulleys so that it rubs against the metal. You can also get this if your chain is way too long, but it looks tight on the bottom. If it’s way too short, you can also get this by trying to force a gear that the derailleur can’t fit, so the chain downshifts or slips off the pulley.