Translation:You try to say thank you to her.
I though "với" was used as "with"
với here is short for đối với = when dealing with, against, towards
To make it read better in English I used You are trying but was marked wrong. I wonder why because again You try to say is really poor English in my opinion as a native speaker.
Read the sentence as though it is a challenge/demand. You* try to say thank you to her.
Why is Bạn thử nói cảm ơn CHO cô ấy wrong?
I too wonder why. Is cho not formally accepted or is this a bug?
In english in fact to say is redundant and "I try to thank her" is a better translation