That's a foobar2000 issue. It needs the executable "flac.exe" to convert files of other types to FLAC. My suggestion is to install the latest version of foobar2000 and see if that fixes the problem. If not, try uninstalling and then reinstalling.
I'm replying again, only to educate any others who may have run into this issue like I did. So, downloading and or reinstalling Foobar2000 didn't fix the problem. Just by trial and error, I fixed the problem, though I really don't understand what got me to the original problem...
In Foobar, when you select "convert", then choose "Flac" - you can right click on the Flac choice, which opens another window that shows "Commandline Encoder Settings - Editing Preset". At bottom of that window it says "Flac encoder 1.2.0 or newer is required", along with a button to visit Flac WEbsite. Well, I did have 1.2.0 or newer, always have. Anyhow, I went to website, downloaded most recent version.
Fire up Foobar again, load my iso file, convert....again, asks me for flac.exe destination (which it never used to do), now, I go to the newly downloaded Flac .exe file I had....and it started conversion.
Close Foobar, restart it. Take another iso file, convert and now it works without me pointing it to the flac.exe
I don't understand what happened in the first place, and now that it works again...I don't really care.