*There are two slots of RAM in that AIO, and I tried both slots as well with no avail.
You mention getting two short beeps, typically a sign of bad RAM and being that the computer is an AIO the graphics rely on the RAM for support.
It is looking like bad RAM but can I ask if you have tried removing the CMOS battery from the motherboard for five minutes or so, just in case of a bad setting in the BIOS, you should also inspect the capacitors on the MB to see if any are blown, bulging or leaking.
If clearing the CMOS does not help I can only suggest that you try and borrow a stick of compatible RAM to try or alternatively have the AIO tested by a local tech/tech store who will have compatible RAM close to hand.
As an asides, it s so annoying that people will penny pinch by using an 8GB stick of RAM rather than 2 Xs 4GB sticks and for this very reason, if you had two sticks of RAM and one goes bad you can still use the computer with the one 4GB stick until you get a replacement, another downside of using the one 8GB stick of RAM is that it will only work in single channel mode thus giving you only half of the performance that you have paid for.