Strictly speaking, and in the formal registers, yes, but colloquially dim is fine.
Can you answer here too: "It's not your fault." I have not tried it, but I am curious.
I answered' it was not your fault', which I see should be accepted, It was rejected and the correct solution given as 'it has not your fault', which is nonsense. I have reported this, but it's a bit dispiriting.
.... mae'r bai is in the present tense, so 'it was...' is incorrect as that is in the imperfect tense.
- Nid arnoch chi mae'r bai. - It is not your fault.
There is no entry in the database for this example saying * 'It has not your fault', so I cannot say why Duo would come up with that.
Probably automatic uncontraction from "it's", if there is an option in the database with "it's".
Is this the only way if saying 'it's not your fault'? I am thinking particularly in terms of word order.
It tends to be an emphatic statement, so in Welsh the dim arnoch chi... is put at the front.