Questions & Answers about Bu kez sözünü tutmalısın.
Bu kez means this time or on this occasion.
- bu = this
- kez = time, occasion, instance
In this sentence, it suggests contrast with earlier occasions, often with the feeling of not like before or for once, this time.
Because kez is used for an occurrence/instance, while zaman usually refers to time more generally.
So:
- bu kez = this time, this occasion
- bu zaman is not natural here
A native English speaker might want to translate this time word-for-word with zaman, but Turkish normally uses kez, sefer, or defa in this kind of sentence.