If you see the error -bash: do-release-upgrade: command not found when trying to upgrade your Ubuntu system, it means that the do-release-upgrade command is not available on your system.
The do-release-upgrade command is used to upgrade an Ubuntu system to the next available release. It is included in the update-manager-core package, which is not installed by default on all Ubuntu systems.
To fix the error and be able to use the do-release-upgrade command, you will need to install the update-manager-core package. To do this, run the following command:
sudo apt-get install update-manager-core
Once the package is installed, you should be able to use the do-release-upgrade command to upgrade your Ubuntu system.
Keep in mind that upgrading an Ubuntu system can be risky, as it involves updating many core components of the operating system. It is always a good idea to make a backup of your system before upgrading, in case anything goes wrong.