Why i cant save animation in mp4

I can't save animation in mp4 format, I know I was able to do that before because my older projects dosen't work anymore

import matplotlib.pyplot as plt
import matplotlib.animation as animate


fig,ax = plt.subplots()

def fun (frame):
    ax.plot([1,2,3])

a=animate.FuncAnimation(fig,fun, 5, interval=20)
a.save('tmp.mp4')
#      'tmp.gif' works

Error message

Exception Unhandled
unknown file extension: .mp4

If I try to save animation as 'gif' I get following message

MovieWriter ffmpeg unavailable; using Pillow instead.

Which I also don't understand because to my knowlege I do have ffmpeg installed

'pip list' gives me this:

certifi         2020.11.8
cycler          0.10.0
ffmpeg          1.4
kiwisolver      1.3.1
matplotlib      3.3.3
numpy           1.19.3
Pillow          8.0.1
pip             20.2.1
pyparsing       2.4.7
python-dateutil 2.8.1
setuptools      49.2.1
six             1.15.0

Anyone knows how to fix that?