It just doesn't sound natural in English. We would say: "there are many hotels here", "there are heaps of hotels here", "there are dozens/hundreds of hotels here", but wouldn't really say "tens of hotels". In fact, I don't think I've ever heard it used in many contexts at all.
It sounds perfectly natural in English. It's certainly become more rare over the years. Here's twenty thousand Google results: https://www.google.com.au/search?q=%22there+are+tens+of%22+-%22tens+of+thousands+OR+millions+OR+billions+OR+trillions%22
That's not to say that I think it should be in a default answer in a Duolingo course. It depends what the alternatives are I suppose.
hey, that's a nice search.
but i think antspants is spot on that 'tens of' sounds completely unnatural in english.
a quick google search of the phrase 'there are dozens of' brings over 4 million results. and there are a number of websites for learners of english that teach this preference for 'dozens of'.
google translate also translates 'dozens' as 'puluhan', and notably not 'lusin', which is nonetheless technically the quantity of a 'dozen' or twelve items.
i know 'puluhan' is literally 'tens of' in indonesian, but in this case 'dozens of' is by far the natural translation. i think we as learners are sufficiently savvy to the context for this variation between the languages to dispense with the overly literal translation used here. (though it has generated really useful conversation about the differences between the languages!)
We sometimes hear/see 'hundreds' and 'tens of hundreds' in English, but we don't usually hear/see 'tens'. The translation is freaking people out, so you should probably consider an alternate translation. I think if you used 'dozens', we would get the point as 'dozens' is not a specific number and it is in the same taman bola. ;-)