I am currently working on a Flutter project and I also getting this problem recently

Current youtube player dependency in mobile right now (mostly) won't be able play the video because they put only the id of the video like this
String url = www.youtube.com/watch?v={videoId}

Yesterday I also working on this also while trying to solve this problem, I browsed on youtube to get the embed html tag then I realized it has ?si parameter youtube_embed_html_tag

so apparently we need to provide that parameter a value to get the video loaded right. The ?si parameter can be filled with any string, here's the example

String url = www.youtube.com/watch?v={videoid}?si=j4sdf2ffh

After trying this, the player managed to load the video without any errors

n.b.: Some video may also won't load properly because some video authors may disabled the embed sharing