If the SolidWorks Installation Manager is a blank or empty screen, use "Windows PowerShell" to Unblock files.

1. Close the "SolidWorks Installation Manager" window.

2. Click the "Start"/"Windows" button and start typing Power.

3. Click "Run as Administrator".

4. In the "Windows PowerShell" command window, either type the below line in, or copy this text and RMB (Right Mouse Button) in the window, it should paste directly.
dir -Path "C:\Windows\SolidWorks" -Recurse | Unblock-File

5. Restart "SolidWorks Installation Manager".

Note: If you also have other locations where the "SolidWorks Installation Manager" is located, the file name is "sldIM.exe". Copy the path to where that file is from "File Explorer", and replace it with the text in the quotes.
For example, in my case it would be:
dir -Path "C:\SWInstallationDownloads\2020\SOLIDWORKS 2020 SP05\sldim" -Recurse | Unblock-File
Then do the same with "Windows PowerShell" with that text.