that's a server error...
Either there was a problem with the server that your client's website is hosted on and you need to check again later, or the hosting is so bad that it constantly gives errors, in which case you might want to get better hosting. If your client uses free hosting tell him to get paid one...
Otherwise, you need to have the googlebot visit the site again multiple times until it finds the site working... Oh, and apparently you've not submitted a sitemap to your GSC, so do that ASAP as it can help with the crawling and indexation of the website