You don't mention which plugins you are unable to use.
The python plugins don't get installed in the same way as most other items. Most, if not all python plugins are downloaded as .zip files. Unzip the files, leaving the folder structure intact and simply move or copy the unzipped folder to:
C:\Program Files\Reallusion\iClone 8\Bin64\OpenPlugin for iClone and C:\Program Files\Reallusion\Character Creator 4\Bin64\OpenPlugin for Character Creator.
The folder should then look something like this depending on which plugins you have placed there.:

Start iClone and the plugins should be available as shown below:
Gerry
System: Win 10 Pro (21H2), Asus X99-E WS, CPU i7-5930K -3,5 GHz, 32 GB DDR4 2666-16 RAM, NVidia GTX 1080 Ti GPU - 11 GB VRAM(Driver Studio-536.99), Samsung 850 Pro 512 GB SSD, 6 TB HD storage.