error after update

Looks like an update failed to complete. You will need to attempt a manual update. Detail steps are given in this article:
https://codex.wordpress.org/Updating_WordPress#Manual_Update

After chasing round for a while I worked out that my wp-settings.php had not been upgraded. This file is the one that links to class-wp-taxonomy.php

Ensuring the backups were good before I proceeded, diffed the old and new to check for no local changes, then swapped in the new one and hey presto.

Thanks @emdeedee presto it is!

You saved me from a ton of aggravation. THANKS!