I have HP desktop start the system and give beep and this earor 912-the machine cover has been removed since last system access was authorized

Hi!

Even after resetting the chassis cover the system generate this error. What you can do is:

1. Make sure that the chassis side cover is closed if not fix it as directed in the assembly guide.
2. Ensure that any system access was authorized, check all the hardware ensuring that no hardware has been changed or removed.
3. Then enter the BIOS setup by pressing F10 then select hardware monitor setup and enable the reset case open status.
4. For maximum security be sure to establish a setup password.

Have fun!

*Your vote is highly appreciated. Thanks in advance!

That means someone unauthorized opened the PC case.

It can be reset in the bios under Security heading.