Since do many are struggling to create nsfw videos with grok, here is how I got it working:
1. Download the app and sign in with X
2. Open your settings by clicking on your profile picture in the left sidebar
3. Inside your settings, click on the profile picture
4. Select the birthday you have set for the account you are using grok with
5. Save your changes
If grok still does not generate nsfw, try again with a different image, because grok seems to refuse some images.
If grok STILL does not generate nsfw, try switching over to the web version:
1. Open the web version
2. In the left sidebar, click on your profile picture
3. Go to account
4. Click on the reload button next to "X account"