"He rarely writes to his father."
Translation:Il écrit rarement à son père.
The rule, though I think there are some exceptions, is BAGS: adjectives describing Beauty (joli, beau, laid), AGe (vieux, jeune), and Size (grand, petit, gros) go before the noun. Others go after.
Here is a detailed explanation:
There is a section for frequency adverbs.
Either because of the accents (although I'm pretty sure they just correct you but still mark your sentence as correct), either (and most likely) because you can't replace "papa" with "father", the same way in which duo marks your sentence as wrong if you write "kid" instead of "child(/enfant)". Also I think the placement of "rarement" is correct :)