You could try a complete uninstall and reinstall, as it used to be we included the denoiser in the Corona installation many versions ago (and you say you started from Corona 6), but now the denoiser is installed by NVIDIA in their drivers. Could be you have some old and new version of the denoiser installed as a result of moving from the old 6 to the newer 10 and 11, so starting over clean might help.
Meantime, as mentioned above, the Intel denoiser is in fact much better in terms of final renders. You can't use it in IR, but you are talking of completing a job so presumably for denoising final deliverables, and the Intel AI denoiser does a far better job there and is our recommended denoiser these days. This means you are not missing out by not having the NVIDIA denoiser in the meantime.
