Due to major bugs on QGIS 3.18 I am forced to install the LTR version. But the official instructions here (https://www.qgis.org/de/site/forusers/alldownloads.html) do only explain how to install the latest version.
I am using Ubuntu 20.04.
Update: I followed the advice below and continued installation, now I got this:
sudo apt install qgis qgis-plugin-grass
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
qgis : Depends: python3-qgis (= 1:3.16.5+32focal) but 1:3.18.1+32focal is to be installed
Depends: qgis-providers (= 1:3.16.5+32focal) but 1:3.18.1+32focal is to be installed
Depends: qgis-common (= 1:3.16.5+32focal) but 1:3.18.1+32focal is to be installed
qgis-plugin-grass : Depends: qgis-plugin-grass-common (= 1:3.16.5+32focal) but 1:3.18.1+32focal is to be installed
Depends: qgis-provider-grass (= 1:3.16.5+32focal) but 1:3.18.1+32focal is to be installed
E: Unable to correct problems, you have held broken packages.