Failed Installation of macOS Catalina
There was a known issue with upgrading or installing the OS on Macs with a T2 chip to macOS 10.15.6. Apple have yet to acknowledge the problem but have rolled out (about 3 hours ago) a new update to the software catalog that is reported to fix the problem.
Give it another try after 0000 UTC Thursday Aug 6 as Apple has replaced a wrong installer for most T2 Macs.