こんにちは (Konnichiwa) イ リ ニ,
不便 (Fuben) means “inconvenient” and is the opposite of 便利 (benri) “convenient.”
It is used to express situations that are inconvenient, for example, not having a car, or living in the countryside.
都合が悪い (Tsugō ga warui) on the other hand, is used to express circumstances that are inconvenient. 都合 (Tsugō) means “circumstances” and 悪い (warui) means “bad,” so literally, it means “circumstances (are) bad.”
It is most often used to express personal availability.
For example, if someone were to ask you to go out for coffee with them tomorrow but you were busy, you would say 都合が悪い (tsugō ga warui).
It's kind of similar to “it doesn't suit,” or “I'm busy" in English; it's just a more polite, roundabout way of saying it!
都合が悪い (Tsugō ga warui) is a lot less direct, so it's usually preferred when declining an invitation over 不便 (fuben).
I hope this helped!
Please let me know if you have any other questions.
勉強を頑張ってください！ (Benkyō o ganbatte kudasai!)