Lorenzo-

You have a lot of copies of Python installed, but only an old version of Python 2.7 that does not support PyUnicode_... functions is in the standard path.

You have 4 options:

 1. Install a copy of Python3 in a standard path. The typical command would be 'sudo apt install python3'. This should place a version of Python 3 in your standard path, e.g. /usr/lib. Next, launch Surfice with the -r argument to reset the path. 

2. Open Surfice, choose Preferences and press advanced. This should open up the text file ~/.surfice.ini into a text editor. You will want to set the PyLib parameter to point to the location of your desired version of Python
 PyLib=/usr/lib/x86_64-linux-gnu/libpython3.6m.so

3. Find some way to install a version of Python2.7 that exports PyUnicode_... functions. On MacOS the default 2.7.16 supports these functions. However, I believe Ubuntu only supports up to Python2.7.15. I do not know how to update to a more recent version, but please share if you find a solution.

4. Since Surfice supports both Python and Pascal scripts, use the Pascal scripting engine which does not depend on you Python installation. The Scripting/Pascal menu provides you with examples.