iMac 27” 2017 won’t boot up after installing memory
iMac 27” 2017 won’t boot up after installing additional memory cards. What to do?
iMac 27″ 5K, macOS 11.0
Where did you buy the modules?
Mac are picky and it is best to purchase modules from vendors like OWC or Crucial.
If you did, then recheck to see if you firmly and fully seated the new modules.