The blinking red lights indicate that the input cover is not seated properly–or that it has been removed–and that the computer is powered on.
There is a small switch near the top rear center of the mainboard, and a foam pad on the input cover that touches it. That switch is the chassis intrusion switch. If that foam pad is misaligned or missing, it will not contact the switch and the chassis will appear to be “open” even if the input cover is on.
Likewise if the switch is missing or broken.
If you have an input cover without a foam pad in the right location or a mainboard with a missing chassis intrusion switch, you should contact support.