Apartment is the correct spelling of the word.
Appartment is incorrect and should not be used in English.
Why is Apartment difficult to spell in English?
“Apartment” is often difficult to spell in English because of the fact that in other languages like French and Italian there are two “p” s.(appartment and appartemento)
It is also often tempting to use double “p” because other words in English are spelled with two “p”s. like “appointment” or “appreciative”.
Remember “apartment” is spelled with only one “p”.