Upgrade to 21.04 breaks apt: symbol lookup error: /lib/x86_64-linux-gnu/libapt-pkg.so.6.0: undefined symbol: XXH3_64bits_reset
You have to remove locally compiled libraries by
sudo rm -v /usr/local/lib/libxxhash.a
sudo rm -v /usr/local/lib/libxxhash.so.0.6.5
and then retry running APT by command like sudo apt-get check.
And finally reinstall the above library using normal deb-packages:
sudo apt-get update
sudo apt-get install --reinstall libxxhash0 libxxhash-dev