This is something I've noticed throughout my journey of learning Swedish on Duolingo - the TTS seems to pronounce "s" like "sh" in many places.
In this case, it sounds like 'du får -shitta- på den här stolen' rather than -sitta-. Is this how the language is, or simply an inconvenience with the TTS? If it is the language, is there a general rule?
That's no error, but a feature of the Swedish language. The letter R will, in all but southern dialects, merge with S when preceding it, to produce the Swedish tje-sound. It can merge even across word boundaries if one words ends with an R and the next starts with an S-sound.
This is one of the few things that the new TTS that we're currently A/B testing does worse than the old one. The new TTS instead swallows the R at the ending of får completely, which is done in some dialects, but not in Standard Swedish.
Since it's an A/B test, different people will hear different things. To hear the new TTS, go to http://www.ivona.com/ and pick Swedish Astrid in the menu.
I typed 'för' instead of 'får' and it accepted it without any feedback. I'm assuming the app thinks it's just a typo but considering that 'för' is an actual word with many sometimes confusing uses, maybe it would be wise to add a 'you have a typo' note if possible or at worst reject it as wrong. I know that might fall outside of what the language contributors are able to do though.