It looks like you've downloaded ISOLDE via your normal web browser, and are trying to install it from the downloaded file. While it is possible to do it this way, in almost all cases it's better to do it via ChimeraX's built-in mechanism (Tools/More Tools... in the top menu). That will automatically handle any dependencies on other ChimeraX bundles. 

If for some reason you do need to do it manually via the downloaded files, you'll first need to download and install the ChimeraX_Clipper bundle. To install a .whl file, you'll need to use the command:

toolshed install {file}

The easiest way in practice is to type "toolshed install " and then drag-and-drop the file from your system file browser onto the command line to auto-fill the path and filename.

Hope this helps!

Tristan