lbr20a you're trying to install a later version of GrapheneOS? Doing so is not recommended because older versions are simply not as secure.
Also, not sure where you're getting this older release, but I would definitely say installing something from somewhere other than the GrapheneOS website is a bad idea. The file could have easily been tampered with.
lbr20a fastboot: error: Failed to boot into userspace fastboot; one or more components might be unbootable.
I am not familiar with this error, but after searching on the internet quickly I think this may be because the release you're trying to flash is not a valid or is corrupt.