In exercises that look like the above, where you need to spell a word or write a phrase with words, you'll have a limited number of spaces to fill in. Once all the spaces are filled, the app won't let you add any more letters or words.
This is intentional!
Palteca automatically stops you from adding more letters or words than the correct answer needs.
It's a little signal that if you were about to add another one, your answer would have been wrong — so it gives you a chance to rethink and try a different combination.
If you're stuck, try removing a letter or word you've already placed and see if there's a better option you missed.