Maybe better to take this offline... just to start - what are the prerequisites, just python3 installed via sudo apt-get python3 ?
peter@peter-ThinkPad-T420s:~$ sudo apt-get install python3-venv
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
command-not-found-data diffstat enchant example-content g++-8 geoip-database
gettext gir1.2-geocodeglib-1.0 gir1.2-mutter-4 gir1.2-mutter-5 ifupdown
intltool-debian libapt-inst2.0 libapt-pkg-perl libapt-pkg5.0 libapt-pkg5.90
libarchive-zip-perl libasync-mergepoint-perl libb-hooks-endofscope-perl
libb-hooks-op-check-perl libbind9-161 libboost-date-time1.67.0
libboost-filesystem1.67.0 libboost-iostreams1.67.0 libboost-locale1.67.0
libboost-system1.67.0 libboost-thread1.67.0 libbrlapi0.6
libcapture-tiny-perl libcgi-fast-perl libcgi-pm-perl libclass-accessor-perl
libclass-method-modifiers-perl libclass-xsaccessor-perl libclone-perl
libcpanel-json-xs-perl libcroco3 libdevel-callchecker-perl
libdevel-globaldestruction-perl libdevel-size-perl
libdigest-bubblebabble-perl libdns-export1104 libdns1104 libdns1109
libdynaloader-functions-perl libebook-contacts-1.2-2 libecal-1.2-19
libegl1-mesa libemail-valid-perl libenchant1c2a libevent-2.1-6 libexiv2-14
libexporter-tiny-perl libfcgi-perl libffi6 libfile-find-rule-perl
libfont-ttf-perl libfprint0 libfuture-perl libgeoip1 libgnome-desktop-3-17
libgnome-desktop-3-18 libgspell-1-1 libgutenprint-common libgutenprint9
libgweather-3-15 libhogweed4 libicu63 libimport-into-perl
libio-async-loop-epoll-perl libio-async-perl libio-pty-perl
libio-string-perl libip4tc0 libip6tc0 libipc-run-perl libiptc0 libirs161
libisc-export1100 libisc1100 libisc1105 libisccc161 libisccfg163 libisl19
libisl21 libjson-c3 libjson-maybexs-perl liblinux-epoll-perl
liblist-compare-perl liblist-moreutils-perl libllvm8 libllvm9 liblouis17
liblouisutdml8 liblwres161 libmodule-implementation-perl
libmodule-runtime-perl libmoo-perl libmoox-aliases-perl libmozjs-60-0
libmutter-4-0 libmutter-5-0 libmysqlclient20 libnamespace-clean-perl
libnet-dns-perl libnet-dns-sec-perl libnet-domain-tld-perl libnet-ip-perl
libnettle6 libnfs12 libnumber-compare-perl liboauth0 liborcus-0.14-0
libpackage-stash-perl libpackage-stash-xs-perl libparams-classify-perl
libparse-debianchangelog-perl libpath-tiny-perl libperl4-corelibs-perl
libperlio-gzip-perl libplymouth4 libpoppler85 libpoppler90 libpython3.7
libpython3.7-minimal libpython3.7-stdlib libqpdf21 libreadonly-perl
libref-util-perl libref-util-xs-perl libreoffice-avmedia-backend-gstreamer
librole-tiny-perl libsereal-decoder-perl libsereal-encoder-perl
libsereal-perl libstdc++-8-dev libstrictures-perl libstruct-dumb-perl
libsub-exporter-progressive-perl libsub-identify-perl libsub-quote-perl
libtagc0 libtest-fatal-perl libtest-refcount-perl libtext-glob-perl
libtext-levenshtein-perl libtype-tiny-perl libtype-tiny-xs-perl
libunicode-utf8-perl libusbmuxd4 libvariable-magic-perl libvpx5
libxml-simple-perl libxml-writer-perl libyaml-libyaml-perl lintian
multiarch-support patchutils pppconfig pppoeconf printer-driver-gutenprint
python3-asn1crypto python3.7 python3.7-minimal t1utils ubuntu-software
ubuntu-system-service ubuntu-web-launchers
Use 'sudo apt autoremove' to remove them.
The following additional packages will be installed:
python3.8-venv
The following NEW packages will be installed:
python3-venv python3.8-venv
0 upgraded, 2 newly installed, 0 to remove and 0 not upgraded.
Need to get 6’668 B of archives.
After this operation, 38.9 kB of additional disk space will be used.
Do you want to continue? [Y/n] Y
Get:1
http://ch.archive.ubuntu.com/ubuntu focal-updates/universe amd64 python3.8-venv amd64 3.8.5-1~20.04 [5’440 B]
Get:2
http://ch.archive.ubuntu.com/ubuntu focal/universe amd64 python3-venv amd64 3.8.2-0ubuntu2 [1’228 B]
Fetched 6’668 B in 0s (23.5 kB/s)
Selecting previously unselected package python3.8-venv.
(Reading database ... 194295 files and directories currently installed.)
Preparing to unpack .../python3.8-venv_3.8.5-1~20.04_amd64.deb ...
Unpacking python3.8-venv (3.8.5-1~20.04) ...
Selecting previously unselected package python3-venv.
Preparing to unpack .../python3-venv_3.8.2-0ubuntu2_amd64.deb ...
Unpacking python3-venv (3.8.2-0ubuntu2) ...
Setting up python3.8-venv (3.8.5-1~20.04) ...
Setting up python3-venv (3.8.2-0ubuntu2) ...
Processing triggers for man-db (2.9.1-1) ...