In the past ( v10) I've used stills as movie sequence, by ticking the Import Media feature for "sequence" - however, my new v12 appears to find an "Error" on each still frame, although, I'm not aware I'm doing things differently.

It imports earlier movie files, along with the attached audio...and these play OK on the timeline.

"Help" Import Media (stills) appears to suggest exactly what I'm doing .. . my frames are jpeg (which I chose from the drop-down...as well as trying All Media Files. ). The stills come from a sony camcorder in stills-mode and I'm sure that in v10 these have been accepted over the years.
I may try the same files in v10 - but hope someone knows What's Different, etc.

I did successfully import( to v12), the same files earlier, by not ticking the "Sequence" button.....however, the sequence is treated as stills I guess... far too lengthy and ( even after Grouping ), cannot be shortened enough by CTL+Mouse . but now the "new" sequence feature appears to reject the same files altogether. But I can Import stills separately . . . . I removed any wrong media from the Project so the bin is empty each try.

I'm unclear about the pop-up details that follow choice...it appears to ask for Timecode, which is not appropriate.

Hope that's clear.