renterd.yml question

As shown in the screenshot above, it seems to be a private_key error. Can I just write the 16 characters I want?
“your_private_key must be exactly 40 characters long” I don’t know what 40 characters means. Does it mean I have to use 40 characters?
I don’t know English well, so I’m uploading a translation. I hope you understand if there are any mistranslations.