Hi, yes, I regularly play with the latest versions of Leela on a Mac and with Hiarcs Chess explorer. Indeed, for it to work there are some preliminary steps to do with the lczero executable, otherwise I get the same error message you mentioned:
1) on the Terminal, I have first to start the lczero executable (./lczero). This somehow "initializes" the program.
2) then I gave the "uci" command to the executable, so that it responded "uciok".
3) Close the Terminal.
After that, I was able add the engine to Hiarcs without problems. Steps 1) to 3) are just needed once (that is, once for every new version of the executable of course).
By the way, when I change network (the "weights.txt" file), I'd first remove the engine from Hiarcs, and then add it again, just to be sure.
Hope this works,
Ric