Reinstall macOS

Try booting into Recovery Mode (Command + R) or Internet Recovery Mode (Command + Option + R) while the laptop is connected directly to the router using an Ethernet and a USB-C to Gigabit Ethernet Adapter which will bypass any hard to troubleshoot WiFi issues.

If you still have a problem, then try an SMC Reset and a PRAM Reset (hold the PRAM Reset for two chimes).

If none of this works, then you can try to "Revive" the T2 firmware, but more likely you will need to "Restore" the T2 firmware, but this requires access to another Mac running macOS 10.15+:

Revive or restore an Intel-based Mac using Apple Configurator 2 - Apple Support

If you are having trouble booting/installing macOS from Internet Recovery Mode and the USB installer (assuming you created the macOS USB installer using Apple's instructions), then I think you need to have Apple or an Apple Authorized Service Provider examine the laptop. Most other methods of creating a bootable macOS USB installer will likely fail to work.

How to create a bootable installer for macOS - Apple Support

The Ethernet connection usually provides a faster more reliable connection plus it bypasses any hard to troubleshoot WiFi issues. If you are having issues with Internet Recovery Mode using WiFi, then definitely try an Ethernet connection to eliminate any possible WiFi issues. A PRAM Reset can sometimes help with Internet Recovery Mode since the Mac may get stuck attempting to access the Apple server in the same manner as the last attempt which failed whereas a PRAM Reset would remove any remnants of the last failed attempt to boot to an Apple server and will possibly use a different method/path.