Vapoursynth doesn't exist.

When I try to watch a video via MPV Video Player I get this error message:
Option vf-add: vapoursynth doesn't exist.

Trying to watch the video via those error messages:
[...]
[00007f6ec85c23b0] chain filter error: Too high level of recursion (3)
[00007f6ec8092130] main filter error: Failed to create video converter
[00007f6ec80929b0] main filter error: Failed to create video converter
[00007f6ecc0463c0] main video output error: Failed to create video converter
[00007f6ecc0463c0] main video output error: Failed to compensate for the format changes, removing all filters
[0000563088302130] svpcontrol interface: video 640x360 @30,0, chroma I420
[h264 @ 0x7f6ed8d62e00] get_buffer() failed
[h264 @ 0x7f6ed8d62e00] thread_get_buffer() failed                                                                                                                                           
[h264 @ 0x7f6ed8d62e00] decode_slice_header error                                                                                                                                           
[h264 @ 0x7f6ed8d62e00] no frame!

How do I fix that?
I followed the exact installation steps shown on this page: https://www.svp-team.com/wiki/SVP:Linux
I'm using Ubuntu 20.04.