Memory error, 2 then 7 amber blinks

Hi, I have a SFF Optiplex 5040 that is not posting and the power button is blinks amber 2 times followed by a pause and then 7 blinks. It also turns on as soon as I plug the power cable in and won’t shut off by holding the power button in.

I can see this is a memory error from the troubleshooting for this model, which stacks up as the problem started when I removed the 1 4GB Ram stick it had. I removed it to upgrade an identical system I had to 8GB. When I try other compatible ram, I get the error. I did put the original Ram back in and it worked again.

I have spent a lot of time researching this and every video or post I read says the error can be fixed by removing and reseating the Ram and/or resetting the CMOS. I’ve done this a number of times and I have had no luck. I’ve tried several different Ram sticks and none of them work (they work in other systems so I know they are good).

Any help would be greatly appreciated - I’ve exhausted everything I can think of. I also no longer have the other system with the original Ram that came out of this system so using that is not an option. 

Thanks