I have the same problem during installation of theme just purchased. this sites “help” isnt exactly user friendly. it seems impossible to locate anywhere i can post a question as a new customer. the help topics dont include this issue. sighs…
Regarding how to get help with your purchase – ThemeForest is just a marketplace; for help with your theme, you’ll want to get in contact with the author (developer) of the theme.
There are many ways to contact an author, but the best way is to find the theme’s page on ThemeForest and click on the Support tab. If you need further help contacting them, see this article: https://help.market.envato.com/hc/en-us/articles/203039054-How-to-contact-an-author
That said, if you’re receiving a “503” error while installing a theme, it may be worthwhile to contact your web host first as this sounds like it is potentially on their side.
Hope that helps!