First I ran: chmod +x hello.py ./hello.py and got: Permission denied I thought the 755 bit was enough, but the real issue was the shebang line. If the first line of a script doesn't point to a valid interpreter, the kernel tries to execute the file directly and fails with permission denied. The fix? Add a proper shebang: !/usr/bin/env python3 or point to the exact path: !/usr/bin/python3 Then run chmod +x hello.py again. Now it works. Lesson: Always...