Is this a new (recent) installation or an old installation?
Apparently your repositories are broken, causing git
errors:
git status
is failing in/etc/.pihole
git rev-parse --abbrev-ref HEAD
is failing in/var/www/html/admin
Try the steps suggested here to fix the repositories and try pihole updatechecker
again.
If still fails, you can try to repair your installation with pihole -r
and select repair, but this won't work if your SD card is failing.