Hi,
I was also experiencing errors with using pihole -up and this error:
/opt/pihole/update.sh: line 108: : No such file or directory
This was raised previously in Files missing after pihole update but the issue was closed, because it looked like an issue with incompatible OS. However I was experiencing the same issue on a compatible OS (Raspberry Pi with Debian/Bookworm).
Upon a detailed inspection, the line 108 was source "${setupVars}" but no setupVars were defined anywhere, hence the error. I'm not sure how this seemingly broken file got to this state, maybe via a previous failed upgrade. I have fixed it by replacing the file's content sudo nano /opt/pihole/update.sh with the latest content from pi-hole/advanced/Scripts/update.sh at 0f7803b7753b581ed747eb6398be0c78dbfdc845 · pi-hole/pi-hole · GitHub
Then rebooted the system, and pihole -up started to work again.
Hope this helps anyone running into the same issue.