Probably because Google is actively and frequently banning many Piped or Invidious hosts, and is generally currently at war with “alternative frontends” to YouTube in an effort to make users browse YouTube directly and consume ads there, or buy YouTube Premium. This is in line with their current fight for more ad revenues across their products and services. You probably have to either search for another public instance which isn’t banned (yet) from accessing YouTube, or host your own instance.

We are at war comrade

I’ve had some issues with Invidious being unable to play specific videos once or twice, but the issues tend to go away after a couple of days.

I mean, we know Google and Youtube are both multi-billion-dollar corporations that are absolutely enraged by the idea that there could be any possible way for a commoner pleb like you to NOT have your online experience absolutely crammed with as many privacy violations and hyper-targeted ads as possible. Invidious having to constantly stay ahead of such powerful entities’ attempts to block them out means that some errors and periods of downtime are perfectly excusable- what’s important is continuing to support them in their fight.

It says “the video might be inappropriate for some users”. Afaik those videos never worked because they required login with Google account