You dont say which parrot you use but with parrot security i had the same problem.
There is 2 way to resolve it but i prefer the second one:
First solution: Edit the 3 python script located at /usr/bin/gns3 /usr/bin/gns3server /usr/bin/gns3vmnet and replace the first line by #!/usr/bin/python3
2nd solution:
make a backup of /usr/share/gns3/gns3-server/bin/python3 and /usr/share/gns3/gns3-gui/bin/python3 like that:
sudo mv /usr/share/gns3/gns3-server/bin/python3 /usr/share/gns3/gns3-server/bin/python3.bak
sudo mv /usr/share/gns3/gns3-gui/bin/python3 /usr/share/gns3/gns3-gui/bin/python3.bak
After create a symlink for python3 in the precedent folder like that:
sudo ln -s /usr/bin/python3 /usr/share/gns3/gns3-gui/bin/
sudo ln -s /usr/bin/python3 /usr/share/gns3/gns3-server/bin/
And after thats use the gui launcher of gns3 in the menu and all works fine 
@no0b i think he make like me install by the official site