Debian deleting a file (Operation not allowed). What to do?

Question

Debian

I created an Onlyoffice swap file in /, but I can not delete it, it says "Operation not allowed".
Selinux disabled, lsof, fuser give empty output.
ls -il /onlyoffice_swapfile

3290 -rwxrwxrwx 1 root root 6442450944 Aug 26 14:23 /onlyoffice_swapfile

lsattr output:

--------------e---- /onlyoffice_swapfile

Removing rm -f /onlyoffice_swapfile

rm: cannot remove '/onlyoffice_swapfile': Operation not allowed

sudo find . -maxdepth 1 -inum 3290 -delete

find: failed to delete './onlyoffice_swapfile': Operation not allowed

Answer 1

https://linuxize.com/post/create-a-linux-swap-file...
sudo swapoff -v /swapfile