Ubuntu reports low disk space, but I can't find where it is [duplicate]

I have Ubuntu running on a virtualbox VM. When I start Ubuntu, it reports "Low Disk Space". But when I check my partitions, I don't see where. I have a 2GB Swap partition and / is a 14GB partition with 2.5GB free space. When I run an update, it is as:

sudo apt-get update && sudo apt-get -y upgrade && sudo apt-get clean && sudo apt-get autoremove

I've also tried using bleachbit, but I still get the error.

probably your /boot has too many kernel versions

first get all kernel list:

dpkg --get-selections | grep "linux-image-[[:digit:]].*" | tr "\t" ";" | cut -d ";" -f1

the result would be sth like:

linux-image-5.3.0-28-generic
linux-image-5.4.0-62-generic
linux-image-5.4.0-65-generic
linux-image-5.4.0-66-generic

then delete just old kernels:

sudo apt-get purge linux-image-5.3.0-28-generic

and delete unused packages:

sudo apt-get autoclean && sudo apt-get autoremove