Learn Finnish: Metsässä minä voin viettää aikaa stressittä ja huoletta. - In the forest I can spend time without stress and without worry.

You add the abessive ending -tta or -ttä to the stem of the word. Since stressi is a modern loanword, its stem is just stressi-. Because the word contains front and neutral vowels (e and i), vowel harmony rules mean we must use the front vowel ending -ttä, resulting in stressittä.

Why is the word for 'without worry' huoletta instead of huolitta?

The basic dictionary form of the word is huoli (worry). For many older Finnish words ending in -i, the -i changes to an -e- to form the stem before adding case endings. This makes the stem huole-, so adding the abessive ending gives us huoletta.

I learned earlier that the word for 'without' is ilman. Could I say ilman stressiä ja huolta instead?

Yes, absolutely! In modern spoken Finnish, it is actually much more common to use the preposition ilman followed by the partitive case (ilman stressiä ja huolta). The abessive case (-tta or -ttä) is relatively rare in everyday speech and is usually saved for written Finnish, poetry, or fixed expressions.

Why is the word for time (aika) in the partitive case (aikaa) here?