You didn't use unzipper to unzip, because if you do, you will see the img.xyzWell maybe it's a good idea to download again, maybe corrupted download.
To mount, simply click on mount at the TWRP homepage, check the partitions you would like to mount (system, persist, etc) and that's it.
I still think you should disable the lockscreen, check if device is not encrypted and delete the lockscreen db files on twrp after.