SQLAlchemy: table version already exists. pgAdmin4 fails on Linux Mint 20 and 20.1
Solution
Downgrade SQLAlchemy to an earlier non-conflict version.
At the time of publishing the posts that explain the steps to install pgAdmin4 within virtualenv on Linux Mint 20, pip was implementing SQLAlchemy 1.3.23. Then, in these last days, the version used is SQLAlchemy 1.4.1
Downgrade can be done with pip install --upgrade.
--upgrade can be used for both, downgrade or upgrade.
(venv) mint@mint:~/pgAdmin4$ pip install --upgrade SQLAlchemy==1.3.23

After this, when trying to run pdAdmin4, a new problem will be displayed:
Traceback (most recent call last):
  File &quot;venv/lib/python3.8/site-packages/pgadmin4/pgAdmin4.py&quot;, line 94, in &lt;module&gt;
    app = create_app()
  File &quot;/home/mint/pgAdmin4/venv/lib/python3.8/site-packages/pgadmin4/pgadmin/__init__.py&quot;, line 347, in create_app
    if not os.path.exists(SQLITE_PATH) or get_version() == -1:
  File &quot;/home/mint/pgAdmin4/venv/lib/python3.8/site-packages/pgadmin4/pgadmin/setup/db_version.py&quot;, line 19, in get_version
    return version.value
AttributeError: 'NoneType' object has no attribute 'value'

This new issue is resolved by removing the ~/.pgadmin directory (created during installation) and then rerunning &quot;python pgAdmin4.py&quot;