frappe.email.queue.flush
redis.exceptions.DataError: Invalid input of type: 'NoneType'. Convert to a bytes, string, int or float first.
just do this command in ur terminal :
env/bin/pip install redis==2.10.6
env/bin/pip install -r apps/frappe/requirements.txt
env/bin/python3 -m pip install --upgrade pip
env/bin/pip3 install redis==2.10.6