Surely you remember the "crash" that YouTube had in 2018 where he could not see any video for about an hour.
Although YouTube does not explain specific reasons to the public, how to fix error 503 often occurs when large numbers of website visitors try to access the website and the server on that site is overloaded to process traffic.
The main cause of YouTube error 503
The site server is simply not available when the server is overloaded and cannot handle the high volume of traffic.
The most recommended thing to do in these cases is to reload the page in addition to constantly checking your Internet connection or restart your router and, later, your computer or device, especially if you see the error message "Service not available - Failure DNS ".
Upload the video after a while as this is a common error message on very popular websites like YouTube when huge increase in traffic overwhelms servers.
The correct way to fix YouTube error 503 is uncertain since many times the causes are extraordinary that the only thing we can do is wait.