What about permissions for the series and season?
Using Young Sheldon as you did:
[v3.0.3.688] System.UnauthorizedAccessException: Access to the path "/downloads/Young.Sheldon.S03E10.Teenager.Soup.and.a.Little.Ball.of.Fib.720p.AMZN.WEB-DL.DDP5.1.H.264-NTb.mkv" or "/tv/Young Sheldon/Season 3/Young Sheldon - S03E10 - Teenager Soup and a Little Ball of Fib WEBDL-720p.mkv.partial~" is denied.
Since Sonarr go far enough to try an copy the file the issue is likely with the destination, /tv/Young Sheldon/Season 3/ in this case.