Besides the very direct text in the error itself …
This means you did not manage your pacnews.
You can further investigate by, ie
man pacman.conf
where you will notice SyncFirst is not listed as a valid option.
So the ‘answer’ is you should remove that option from your /etc/pacman.conf
(and/or additional files in /etc/pacman.conf.d/)
Which would have been apparent from /etc/pacman.conf.pacnew that would have been generated during the package upgrade.
It would have been printed out during the upgrade as well. ex
[ALPM] warning: /etc/pacman.conf installed as /etc/pacman.conf.pacnew
In case an introduction to pacnews is needed …