From what you’ve posted, you’re missing:
For Seafile 7.1.x
# on Debian 10/Ubuntu 18.04
apt-get update
apt-get install python3 python3-setuptools python3-pip -y
pip3 install --timeout=3600 Pillow pylibmc captcha jinja2 sqlalchemy django-pylibmc django-simple-captcha python3-ldap
(https://download.seafile.com/published/seafile-manual/deploy/using_mysql.md)
Also, the guide you’re using mentions libpython3.8 instead of 3.6 for Ubuntu 20.04.
Another way to get more information is using
./seahub.sh start-fastcgi
While fastcgi is discontinued, this gives you more details on what’s going wrong.