That letter was not written by me
Both is correct
Surat-Letter Tu/itu-that/those tak/tidak-is not/not ditulis-written olehku-by me
You could also say this: "Surat itu tidak ditulis oleh saya/aku." Which does mean the same thing.
I am surprised to see "ku" attached to a preposition?
It's true tho
What is the difference between ditulis and menulis?
Suggest that 'non-passive voice' translation should be accepted and shown. In this case a more natural way to say in English would be "I did not write that letter".
To be fair, the point of this section is to demonstrate how the passive is formed in Indonesian, so anyone removing the passive voice in their translation would miss out on learning that.