OK, I got the solution.
1. Remove any Python3 you have installed in the DSM.
2. as you have Python3.7.7 in domoticz just add the bin folder to the PATH:
Edit etc/profile file and add to PATH "/volume1/@appstore/python3/usr/bin/", it should look like that
"PATH=/volume1/@appstore/domoticz/bin:"
To apply the above run the command: sudo source /etc/profile
Now you can type python3 or python3 -V in terminal, it should run python3.7.7 or show you the version 3.7.7 if you used python3 -V command
Now you can install pip (curl -k
https://bootstrap.pypa.io/get-pip.py | python3) and all you need....
My domoticz log doesn't show any more errors related to python anymore.
Also I can run any python.py scripts from the cron using Python3.7.7
PS. for the cron I had to edit crontab file (etc/crontab) and add python Path "PATH=/volume1/@appstore/domoticz/bin"