Is this a new (recent) installation or an old installation?
Apparently your repositories are broken, causing git errors:
git statusis failing in/etc/.piholegit rev-parse --abbrev-ref HEADis 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.