Make sure your 27-inch iMac recognizes its new memory
After you install memory in your 27-inch iMac, check that your iMac recognizes the new memory. See Confirm that your iMac recognizes its new memory (scroll to the final section of the Apple Support article).
Start up your iMac.
When you see the macOS desktop, choose Apple menu > About This Mac.
You’ll see the total amount of memory installed in your iMac. For a detailed view of the amount of memory installed, click System Report, then click Memory on the left under Hardware.
If the memory you installed doesn’t meet specifications, your iMac will sound three tones every five seconds. If this happens, shut down your iMac and check the installation instructions again to make sure that the memory is compatible and installed correctly. If you still have problems, remove the memory and consult the support information that came with the memory or contact the vendor who provided the memory.