"Thank you, goodbye."
Translation:Děkuji, na shledanou.
We call them diacritics. Accents are in French, for example.
It depends. Are you using the Duo app on your phone? Most phones allow you to type characters with diacritics when you hold a letter - e.g. hold "A" and you'll be able to choose from Á, Ä, Å and more. In case this doesn't work, install a different keyboard on your phone.
Are you using the web version on your computer? Then you have two options: Install a Czech keyboard layout. On Windows, find the Language control panel, and under Keyboards and Languages add another layout (CZ). The other option is to use the little buttons with "special characters" that Duo provides under text input boxes - but then you have to click on them with your mouse in between typing, so it's not comfortable.
The last option is to type without diacritics, using the plain latin alphabet. Your answers will still be accepted, but it's not great for learning.
"Na shledanou" is the formal way of saying goodbye. You would use it when departing from a stranger or a person of authority, etc. It is a respectful goodbye - I would even say it is more respectful than "see you later" is in English.
The full meaning of the phrase is something like "may we meet again".
For now, it is just good to remember it as a phrase, as that's how it is regularly used.