"She is not satisfied with the service at the hotel."
Translation:Hon är inte nöjd med servicen på hotellet.
'service' in the English sentence refers to something general, but tjänst is very singular and specific, more like a favor which it can also mean.
a service can be en tjänst when it's specific, like for instance when your ISP delivers your internet connection to you: that is a service in English and en tjänst in Swedish. But the service at the hotel is a more general thing, a whole set of services that they perform for you. Hope this helps, it's a bit hard to explain!