Unable to launch vehicle in Quick start

[alliWong]

Hello,

I am following the https://uuvsimulator.github.io/quick_start/ tutorial. i was able to successfully launch the gazebo world, but I am unable to launch roslaunch uuv_descriptions upload_rexrov.launch mode:=default x:=0 y:=0 z:=-20 namespace:=rexrov, having the following error Traceback (most recent call last):
File "/home/allison/Workspace/AuvWs/src/uuv_simulator/uuv_descriptions/scripts/spawn_model", line 41, in
import tf.transformations as tft
File "/opt/ros/melodic/lib/python2.7/dist-packages/tf/init.py", line 30, in
from tf2_ros import TransformException as Exception, ConnectivityException, LookupException, ExtrapolationException
File "/opt/ros/melodic/lib/python2.7/dist-packages/tf2_ros/init.py", line 38, in
from tf2_py import *
File "/opt/ros/melodic/lib/python2.7/dist-packages/tf2_py/init.py", line 38, in
from ._tf2 import *
ImportError: dynamic module does not define module export function (PyInit__tf2)
[rexrov/urdf_spawner-1] process has died [pid 7388, exit code 1, cmd /home/allison/Workspace/AuvWs/src/uuv_simulator/uuv_descriptions/scripts/spawn_model -urdf -x 0 -y 0 -z -20 -R 0.0 -P 0.0 -Y 0.0 -model rexrov -param /rexrov/robot_description __name:=urdf_spawner __log:=/home/allison/.ros/log/d21bc49a-0273-11eb-8f20-9061aef9286f/rexrov-urdf_spawner-1.log].
log file: /home/allison/.ros/log/d21bc49a-0273-11eb-8f20-9061aef9286f/rexrov-urdf_spawner-1*.log
`
My system:
Melodic
Gazebo-9
Ubuntu 18.04

I have build my system with both python 2.7 and 3.6, but I'm still unable to launch the vehicle...(both with the same error)