Also, did you check if the boinc client and manager are available in the repositories? Looking at the manual for Linux Lite, it uses Synaptic Package Manager. So you could start Synaptic and search for boinc.

https://www.linuxliteos.com/manual/software.html#instsoftware

Using the Synaptic Packager Manager, I found various entries for BOINC.

- Marked the first package boinc 7.6.31+dfsg-6ubun metapackage for the BOINC client and manager, for installation.
- Said yes to the dependencies which then caused the following packages to be selected: boinc-client, coinc-manager, libboinc7.
- Clicked Apply then Yes.
- An error occurred.
E: Could not get lock /var/cache/apt/archives/lock - open (11: Resource temporarily unavailable)
E: Could not open file descriptor -1
E: Unable to lock the download directory.

Stymied again.

This isn't very nice what the installation is doing (if you don't get any message that something went wrong along the way).
"sudo: unknown user: boinc" means that the user "boinc" is missing in the password file.
The installation ("sudo apt-get install boinc-client boinc-manager") should have created a user boinc (if it isn't there yet). It also means that all BOINC programs and directories aren't owned by user boinc at the moment and that is a bigger problem, so I suggest that you uninstall everything, start over and watch where it goes wrong. What is the output of "sudo apt-get install boinc-client boinc-manager"?
Or is there anybody who can tell that it is normal that user boinc is missing on Ubuntu? :-(

On my Fedora Linux installation one of the first scriptlets says that a user boinc is being created like this:
preinstall scriptlet (using /bin/sh):

# Create BOINC user and group
getent group boinc >/dev/null || groupadd -r boinc
getent passwd boinc >/dev/null || \
useradd -r -g boinc -d /var/lib/boinc -s /sbin/nologin \
    -c "BOINC client account." boinc
exit 0

Warning: Don't use this as is on your system.

sudo apt-get install boinc-client boinc-manager
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
libboinc7 libjavascriptcoregtk-1.0-0 libwebkitgtk-1.0-0
libwebkitgtk-1.0-common libwxbase3.0-0v5 libwxgtk-webview3.0-0v5
libwxgtk3.0-0v5
Suggested packages:
boinc-client-opencl boinc-client-fglrx boinc-client-nvidia-cuda
The following NEW packages will be installed:
boinc-client boinc-manager libboinc7 libjavascriptcoregtk-1.0-0
libwebkitgtk-1.0-0 libwebkitgtk-1.0-common libwxbase3.0-0v5
libwxgtk-webview3.0-0v5 libwxgtk3.0-0v5
0 upgraded, 9 newly installed, 0 to remove and 40 not upgraded.
E: Could not get lock /var/cache/apt/archives/lock - open (11: Resource temporarily unavailable)
E: Unable to lock directory /var/cache/apt/archives/