i want to install openStack on ubuntu 16.04. when i enter the command ./stack.sh i get the following error
python version is 2.7.12 , i also read the openstack.org documentaion, it said in Prerequisites that use to up version 2.7.

Ignoring zipp: markers 'python_version == "2.7"' don't match your environment
Ignoring zipp: markers 'python_version == "3.6"' don't match your environment
Ignoring zipp: markers 'python_version == "3.7"' don't match your environment
Ignoring zipp: markers 'python_version == "3.8"' don't match your environment
Obtaining file:///opt/stack/keystone
ERROR: Package 'keystone' requires a different Python: 3.5.2 not in '>=3.6'
+inc/python:pip_install:1                  exit_trap
+./stack.sh:exit_trap:489                  local r=1
++./stack.sh:exit_trap:490                  jobs -p
+./stack.sh:exit_trap:490                  jobs=
+./stack.sh:exit_trap:493                  [[ -n '' ]]
+./stack.sh:exit_trap:499                  '[' -f '' ']'
+./stack.sh:exit_trap:504                  kill_spinner
+./stack.sh:kill_spinner:399               '[' '!' -z '' ']'
+./stack.sh:exit_trap:506                  [[ 1 -ne 0 ]]
+./stack.sh:exit_trap:507                  echo 'Error on exit'
Error on exit
+./stack.sh:exit_trap:509                  type -p generate-subunit
+./stack.sh:exit_trap:510                  generate-subunit 1589480494 258 fail
+./stack.sh:exit_trap:512                  [[ -z /opt/stack/logs ]]

Each time I get error, I re-enter the following commands:
./unstack.sh
./clean.sh
./stack.sh
But I get the same error again.