I usually see the word written in katakana, although a kanji term is available, and wanted to know the reason.
だめ in the sense of "no good", "not allowed", etc. derives from 駄目, which is a term from 囲碁 igo "Go" (the game).
This derivative meaning may also be written as 駄目 (both characters are jōyō kanji). However, it appears more frequent in kana. The frequencies are given as follows:
ダメ 5279 44% だめ 4247 35% 駄目 2565 21%
Katakana are often used for emphasis, or simply for using a different script in order to make the word visually distinct from the rest of the sentence (which might be in hiragana as in これはダメです).
In any case, kana are far more common. One reason might be that ダメ・だめ are used for the derivative meaning of "no good" or "not allowed" to distinguish it from the literal (game of Go) meaning, which would primarily be written 駄目.
Do you mean you think it should be written in kanji? Or katakana?
だめ is a Japanese-origin word. It's written as 駄目 in kanji, and this is a mixed on-kun compound. Almost all adults understand 駄目 written in kanji.
It's often written in hiragana or katakana because its kanji are relatively difficult and unrelated to the current main meaning of 駄目. See my previous answer for details and similar examples.