"I didn't prepare!"

Translation: 我没有准备！

[PrestonSem]

Why don't you need a 了？

[ricky._.turner]

Would 我不准备了 work?

[chetlin]

No, the negative of verb + 了 is 没有 + verb. You can't keep the 了 and just add 不.

[golvok]

An interesting thing to say... seems to be literally "I don't have ready(ness)". I wonder if this is 'one of those things that people say to mean this' (ie. an idiom). Also, is this a valid (if maybe weird) way of saying "I don't have a plan" or "I don't have preparations"?

[Nanani6]

I don't think it counts as an idiom in Chinese, no. It's just that "being ready" isn't a verb like it is in English. Readiness (or preparations if you prefer, but not plan) is a thing that you have.
Different ways of thinking of the same concept in different languages.

[Carmen_Yomisma]

Would then "I'm not ready" be a more accurate/direct translation? In the sense of translating a substantive for another and so keeping the structure of the Chinese sentence?
准备 is one of the concepts that is becoming more difficult for me to understand in this course

[Languagecr10]

Does this sentence refers to "study" preparation or "dressing"?