I can’t connect to practice range either in the card selection or simply in custom game (any other mode works), to try Venture.
I’d imagine new hero means servers getting over worked.
An outside option, you could have something downloading in the background. I had it yesterday where it just would not load into the practice range. Turned out steam was updating a game in the background like a cheeky little scamp.
Started to work again so probably a server issue 
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New hero buzz. Standard for blizzard to not prep servers for it.