Well, no crap, the update only explicitly say "REQUIRES 14+". If you skipped 14.X (and you did), you can't go straight for 15. You gotta download the whole thing again. I'd suggest asking around Discord, folks MIGHT (just to add emphasis, MIGHT, as in, not guaranteed in any way shape or form, but possible if you are lucky) still have that somewhere tucked around in their hard drive if you don't want to download the whole 15, and just want to upgrade to 14, then 15.
Viewing post in Harem Hotel (18+) NEW UPDATE! comments
From what I understand they downloaded the whole game when it was in v0.13.2 and the patch framework was introduced.
Then they applied the patch to get it to v0.14 and it worked fine (so unless there's a difference between a patched v0.14 and a fresh one it should be fine to apply the v0.15 patch on top of it).
Finally comes the time to install the v0.15 patch and it doesn't work anymore. I can confirm that happens, Ren'Py gives an error.
The whole v0.15 download works though so I'm not sure what's wrong with the patch files (other than having a non-executable Harem_Hotel.sh, but that's easily fixed).