OWC products QC are terrible. Better do RMA with them.

If you put in stock RAM and the same slot reporting error then it could the slot itself problem. But in most cases the slot just dusty as it is not in use for many years. Try cleaning it and reinsert few time it might solve the problem.

Don't use any liquid. It's high risk. Just brush and blow the slot gently. Re-insert the memory few time. Sometime it will work by trying only one slot. Forcing the Mac to work on that particular slot.