You can change your post preferences as follows.
From All-Ages to R-18
1. From the menu, open your Creator Preferences
Go to "Posting R-18 contents" and click "Edit"
3. Check "Yes" and click "Save changes"
From R-18 to All-Ages
The pixivFANBOX team can assist you in changing your settings to disable the posting of R-18 content.
You can learn more in the article below.