After installing python3.8-venv as dependency from python3-venv there is a symlink from /usr/bin/pyvenv, but no pyvenv-3.8 seems to exist.

ii python3-venv 3.8.2-0ubuntu2 amd64 pyvenv-3 binary for python3 (default python3 version)
ii python3.8-venv 3.8.10-0ubuntu1~20.04.7 amd64 Interactive high-level object-oriented language (pyvenv binary, version 3.8)

Could run:

sudo updatedb

Then use the locate command to find the binary

# locate pyvenv-3.8
/snap/core20/1518/usr/share/bash-completion/completions/pyvenv-3.8
/usr/share/bash-completion/completions/pyvenv-3.8

# ls -al /usr/share/bash-completion/completions/pyvenv-3.8
lrwxrwxrwx 1 root root 6 Feb 2 2020 /usr/share/bash-completion/completions/pyvenv-3.8 -> pyvenv
# ls -al /snap/core20/1518/usr/share/bash-completion/completions/pyvenv-3.8
lrwxrwxrwx 1 root root 6 Feb 2 2020 /snap/core20/1518/usr/share/bash-completion/completions/pyvenv-3.8 -> pyvenv

# ls -la /usr/bin/pyvenv
lrwxrwxrwx 1 root root 10 Mar 13 2020 /usr/bin/pyvenv -> pyvenv-3.8

# which pyvenv-3.8
(no result)

apt-cache showpkg python3.8-venv
Package: python3.8-venv
Versions:
3.8.10-0ubuntu1~20.04.7 (/var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_focal-updates_universe_binary-amd64_Packages) (/var/lib/apt/lists/security.ubuntu.com_ubuntu_dists_focal-security_universe_binary-amd64_Packages) (/var/lib/dpkg/status)
 Description Language:
                 File: /var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_focal_universe_binary-amd64_Packages
                  MD5: d16828bf8bf56928e6bfb6f1a9a2f48a
 Description Language: en
                 File: /var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_focal_universe_i18n_Translation-en
                  MD5: d16828bf8bf56928e6bfb6f1a9a2f48a
 Description Language:
                 File: /var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_focal-updates_universe_binary-amd64_Packages
                  MD5: d16828bf8bf56928e6bfb6f1a9a2f48a

3.8.2-1ubuntu1 (/var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_focal_universe_binary-amd64_Packages)
 Description Language:
                 File: /var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_focal_universe_binary-amd64_Packages
                  MD5: d16828bf8bf56928e6bfb6f1a9a2f48a
 Description Language: en
                 File: /var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_focal_universe_i18n_Translation-en
                  MD5: d16828bf8bf56928e6bfb6f1a9a2f48a
 Description Language:
                 File: /var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_focal-updates_universe_binary-amd64_Packages
                  MD5: d16828bf8bf56928e6bfb6f1a9a2f48a

Reverse Depends:
  python3.8-full,python3.8-venv 3.8.10-0ubuntu1~20.04.7
  python3-venv,python3.8-venv 3.8.2-1~
  python3.8,python3.8-venv
  python3.8,python3.8-venv
Dependencies:
3.8.10-0ubuntu1~20.04.7 - python3.8 (5 3.8.10-0ubuntu1~20.04.7) python-pip-whl (2 8.1.0-2) python3.8-distutils (0 (null)) python3-pip (3 1.5.6-4)
3.8.2-1ubuntu1 - python3.8 (5 3.8.2-1ubuntu1) python-pip-whl (2 8.1.0-2) python3-pip (3 1.5.6-4)
Provides:
3.8.10-0ubuntu1~20.04.7 - python3.8-venv:any (= 3.8.10-0ubuntu1~20.04.7)
3.8.2-1ubuntu1 - python3.8-venv:any (= 3.8.2-1ubuntu1)
Reverse Provides:

It seems that you experience the problem that has already been reported as Bug #1879188

Please also note the remarks in https://askubuntu.com/q/1304641
https://docs.python.org/3/whatsnew/3.6.html#whatsnew36-venv

Thank you for your helpful comment.