The system check is designed to ensure the computer's BIOS or UEFI firmware is compatible with BitLocker and that the TPM is working correctly. The system check can fail for several reasons:
The computer's BIOS or UEFI firmware can't read USB flash drives.
The computer's BIOS, uEFI firmware, or boot menu doesn't have reading USB flash drives enabled.
There are multiple USB flash drives inserted into the computer.
The PIN wasn't entered correctly.
The computer's BIOS or UEFI firmware only supports using the function keys (F1-F10) to enter numerals in the pre-boot environment.
The startup key was removed before the computer finished rebooting.
The TPM has malfunctioned and fails to unseal the keys.