Why "Katerina is buying for her husband a new suit" is not accepted?
That is a very nice literal translation, but it is unlikely to be used in speech or in writing. It is not grammatically incorrect, but it's not the normal word order for native English speakers.
Kateřina buys a new suit for her husband. Accepted (now). It means she is gone away, shopping. But you don't see her doing the action. (There's no necessity for it to be an habitual action)
I wrote "Katerina is buying for her husband a new suit" and it says it's not correct. It should be "Katerina is buying a new suit for her husband". Please add this option as correct as well.