You've posted in the Lounge, a non-technical forum for creative conversation. I'm guessing you are using Lightroom, so I'll move this to a Lightroom product forum where more people will see it.
''Have you signed in to Lightroom on the new iMac to activate it? The Develop module is unavailable if you don't have a current subscription. Sign in with your Adobe ID by clicking the Identity plate in the upper left corner and choosing the option to sign in.
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I also observed that in the develop mode a note "image couldn't be found". This may have been a result of my computer guy having just copied all the files from my old computer to the new one?
It would help if you stated the version number of Lightroom that you are using.
It's possible that the photos are not in the location Lightroom expects them to be in. Are there question mark icons or exclamation point icons on the photos (see example here: Adobe Lightroom - Find moved or missing files and folders ) If so, follow the instructions in that article to re-connect the photos in Lightroom.
DJ has you pointed in the right direction for finding missing photos.
Check this link to make sure that you got all the needed files moved to the new computer.