"František always agrees with her."
Translation:František s ní vždy souhlasí.
Is there a particular rule around ordering in Czech that makes one more natural than the other?
- František vždy souhlasí s ní: this was marked incorrect
- František s ní souhlasí vždy: this was the suggested correction in response
"František vždy souhlasí s ní" strongly stresses "s ní" and it sounds weird. It is hard to draw a line though. Perhaps it will become accepted. The problem is that flat out acceptance does not warn the user about the uncommonness.
"František s ní souhlasí vždy" Here "s ní" is in the second position, which is unstressed. We normally place clitics there. It is a common word order.
Basically, the English sentence is ambiguous. It does not say what is already known and what is the new information and how the sentence is intonated. If you want to stress that he always agrees with her and not to someone else it is probably alright.
If you want to say that he always agrees (with her) then your answer is incorrect.