delete folder fail with root account in ubuntu [closed]

I am running ubuntu22.04 on virtualbox hosted on windows11. I have a folder with nothing in it and delete it fail with root account.

drwxrwx--- 1 root vboxsf 4.0K Apr 16 12:55 pngroot@ub2204:# rm -rf pngrm: cannot remove 'png': Operation not permitted

What's the reason root user can not delte a folder? How can I delete this folder?

rmdir not work either:

root@ub2204:# rmdir png/rmdir: failed to remove 'png/': Operation not permitted