i posted this here also github.com/inkcut/inkcut/issues/387
i had the same problem. frmdstryr is correct. you need to get PyQt5 into the pipx virtual environment inkcut folder which is like a container that contains all dependencies and apparently isn't looking elsewhere for missing dependencies (has a requirements.txt document that it references and pulls them in; which you don't see (it is in the source code folder). option 1 'sudo apt-get install' puts PyQt5 in your /usr/lib/python3/dist-packages folder and option 2 has it's own venvs location; but 'pipx install inkcut' installs to it's own venvs folder. but didn't seem to pull in PyQt5.
so here's what i did and inkcut opens and seems to be working but my plotter is in storage so i haven't gotten to test it yet:
1) from github issue #387 above it seems you already have pipx in your PATH (if not add pipx to your PATH in terminal with 'pipx ensurepath' minus the quotes)
2) copy PyQt5 folder from /usr/lib/python3/dist-packages, go to /home/yourusernamehere/.local/share/pipx/venvs/inkcut/lib/ open whatever python folder is there (mine's python3.12) open 'site-packges' folder and paste PyQt5
3) open terminal run 'inkcut' (minus the quotes) but it'll close if you close the terminal
to run inkcut without having to keep the terminal open go to /home/yourusernamehere/.local/share/pipx/venvs/inkcut/bin and double click inkcut file.
make a desktop or menu launcher if so inclined.