Difficulty transferring data from iMac to Mac mini

Even though I loved the all-in-one design of my iMac 2013, I recently broke down and purchased a Mac mini. My issue is I can’t seem to transfer files from my iMac to mini. I purchased a thunderbolt connector at the suggestion of both the sales agent at Best Buy and what it recommends in the Mac mini info, but it doesn’t fit into my iMac. I tried to circumvent this by attempting to download with iCloud, but when I click on migration assistant, that option doesn’t appear. It simply says it’s looking for other sources, but never does. If I click on continue, it says the selected source cannot be used for migration because the volume contains an installation which may be damaged.

I can’t find the damaged files in disk utility. I am sure that has more to do with my lack of knowledge than anything else, but here I am, stuck!

Because of the nature of my old Mac, I can’t seem

to have the option of a workable mouse on both screens, which I think may be a part of the problem.

Any help would be appreciated.