Ubuntu OpenCV install instructions (not from source) don't work on Rock4c

Installed Ubuntu20 using rockpi4c install instructions.

Using non-source install instructions for OpenCV:

I get this on the last instruction:
sudo apt-get install opencv4.0
Reading package lists… Done
Building dependency tree
Reading state information… Done
Note, selecting ‘libopencv4.0-java’ for regex ‘opencv4.0’
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

The opencv test then fails.