⚠ There is not enough space to download and install the selected items

I tried to change the install location of the Hub but without success. How can I download the Unity Editor?

In the Linux environment, temporary downloads are first stored in the /tmp folder and then moved to the destination directory. Downloads fail to start if the estimated free space for that folder isn't enough, especially when there are system partitions.
User apollongt on Reddit explains a convenient solution to the issue in this thread. I'm assuming the installation directory wasn't changed (the standard directory is ~/Unity/Hub/):
1. Create a folder named tmp in your Home directory (Bash command: mkdir ~/tmp).
2. Navigate to the folder where the UnityHub.AppImage is and open in Terminal (or, Bash: cd ~/Unity/Hub).
3. Execute the following command from the Terminal: TEMP=~/tmp ./UnityHub.AppImage.
The Unity Hub will open, and you'll be able to download the Editor. This way, you're telling the Hub to use ~/tmp in Home as a temporary location for downloads rather than /tmp (note the missing '~', short for the Home directory).