Python 3.12 is no longer included by default in Ubuntu 26.04 LTS, which caused the OpenAdmin service to fail after upgrading.
To fix this you have to manually installPython 3.12 via commands:
sudo add-apt-repository ppa:deadsnakes/ppa
sudo apt update
sudo apt install -y python3.12 python3.12-venv python3.12-dev
sudo systemctl restart admin
After that, admin service should work fine.