I don't think it could be the reason.
There's a workaround, posted by @katerpanne
"TWRP update shows ERROR: 1 / can not mount system ...
- this error can occur if you try the update with updater app or if you started TWRP manually
-> while you are in TWRP reboot your device in recovery mode
-> try to install the rom again
-> if it will fail, reboot in recovery again
-> or use "fastboot boot twrp.img" again
-> sometimes you need more then one try
-> if it will not work choose another twrp version"
(reboot Recovery means to restart TWRP Recovery from TWRP "Restart - Recovery")
The last point means, you can try it with one of the the previous version of Android 12 TWRP, for example:
(You use/have v. 6.3 and this link is v. 6.2 from same TWRP)
And you should check the md5 checksum of the downloaded Rom file. It must match the number, which is written on the download website under "i" (information).
Sometimes downloaded zip files are corrupted.
You can check the zip file for example with 7-zip, right click on the file - check archive file.
If zip file doesn't match the one from the download website, download the Rom again.