"Las niñas podrán escribir cartas."
Traducción:The girls will be able to write letters.
Hi. You cannot modify "can" with the auxiliary/modal "will" When "can" refers to permission, possibility, or suggestion, the Future is "can" + future time adverb, so that it's clear you mean "future" Ex: You can go to the cinema when you finish your exercises I can help you in a little while You can read a book while we are out
When "can" refers to ability/capacity, the future form is "will be able to" Ex: I can't swim very well, but I will be able to swim much better after a few classes.
Hope it helps.
Si no me equivoco, es porque los dos son verbos modales o auxiliares. "Can" solo puede usarse en presente o pasado ("could"), y en futuro se transforma en "be able to".