This seems to be a longstanding problem. I'm getting that "Preview unavailable for this file" message.
Here are the data from Help/System Info:
Lightroom Classic version: 10.2 [ 202103041821-226a1211 ] License: Creative Cloud Language setting: en-US Operating system: Mac OS 10 Version: 10.16.0 [20E241] Application architecture: x64 Logical processor count: 4 Processor speed: 3.5 GHz SqLite Version: 3.34.0 Built-in memory: 8,192.0 MB Real memory available to Lightroom: 8,192.0 MB Real memory used by Lightroom: 229.5 MB (2.8%) Virtual memory used by Lightroom: 7,758.6 MB Memory cache size: 52.1MB Internal Camera Raw version: 13.2 [ 738 ] Maximum thread count used by Camera Raw: 3 Camera Raw SIMD optimization: SSE2,AVX,AVX2 Camera Raw virtual memory: 120MB / 4095MB (2%) Camera Raw real memory: 122MB / 8192MB (1%) Displays: 1) 5120x2880
Graphics Processor Info: Metal: AMD Radeon Pro 575
I had a support guy from Adobe online this morniung. He spent more than an hour tracking down the problem, which he said came about because the newest version of LRC haas differnt permission functions. I had to give the tech access to my computer, and I watched as he went through Security & Priacy/Privacy/Files and Folders to give full access to LRC, PS and Bridge. Upon reestart, the import woerked fine. I'm not entirely sure that's all he did. It took a bit of poking around before he was able to work it out. It was good to actually have the problem solved rather than find a work around.
>he went through Security & Priacy/Privacy/Files and Folders to give full access to LRC, PS and Bridge
That is overkill but will work. You only need to give it access to desktop, downloads, removable volumes, and network volumes in the privacy panel. This is a very poorly documented security feature in Mac OS X where it prohibits new apps from accessing these folders by default. Normally it should pop up a dialog allowing you to give access to the app requesting access to one of the above folders but I've seen it often not do that.