The license file (“license.txt”) is easy to get, so don’t worry if you don’t have an existing license. Just ask follow the link to fill in the form to request another one if you are setting up on a new system.
Use the Ubuntu “DEB installer” download
Before installing the freesurfer download, install the dependencies (https://surfer.nmr.mgh.harvard.edu/fswiki/BuildRequirements ) .
Note: use apt rather than apt-get
It is not unusual for these commands to not work as listed. So, go through these steps from a terminal (“#” is for comments):
...:~$ sudo apt-get -y update # update all installed software # Try this ~$ sudo apt-get -y install git wget tcsh xxd build-essential gfortran # If there are errors, see suggestions in terminal. For example, you might run ~$ sudo apt --fix-broken install # Then repeat install - these are copied from webpage (18 August 2023) ~$ sudo apt-get -y install git wget tcsh xxd build-essential gfortran ~$ sudo apt-get -y install libblas-dev liblapack-dev zlib1g-dev ~$ sudo apt-get -y install libxmu-dev libxmu-headers libxi-dev libxt-dev libx11-dev libglu1-mesa-dev
You can try installing with the Software Install tool, and it will report if there are more dependency problems.
To find the dependencies, go to a terminal (since the software tool does not report what dependencies are missing).
Open a terminal in Downloads and run the command line install:
~$ sudo dpkg -i freesurfer_ubuntu...._amd64.deb # Use file name of downloaded freesurfer file
Look for names of packages not installed, and install those
Example:
~$ sudo dpkg -i freesurfer_ubuntu...._amd64.deb # Use file name of downloaded freesurfer file
Look for names of packages not installed, and install those. This is a hunt. In order to install new packages, yuo’ll need to uninstall freesurfer -
sudo apt --fix-broken install
Then install what you need
sudo apt-get -y install xorg-dev sudo apt-get -y install gettext sudo apt-get -y install csh
Who knows, yuo might want to run these two
sudo apt --fix-broken install sudo apt autoremove
However, we might want to use gdebi because unlike dpkg it resolves dependencies
sudo apt install gdebi
However, this does nto work. We need the libbrotli1 library. So 
https://github.com/google/brotli
sudo apt-get install curl zip unzip tar
From here, we can install the latest brotli, which has that library. However, the package manager does not recognize the updated version. Thus, we are left to abandon the *.deb route.