I can't delete text in transcript

Hi,
I imported audio files and made new caption. It automatically generated transcript as well.

After that, I changed script to text-based editing.

However, even before and after the change, I cannot delete transcript.

I wanted to delete it so that the audio file is cut automatically and I dont have to trim it by myself.

Delete button is active, not greyed out, but even tho I click it, it is not deleted.

What should I do?