There can be several reasons for ETS installations to fail. These are the most common ones:
- Pending Windows updates
- Active antivirus and/or firewall software
- Wrong or incomplete .NET Framework
- Another software installation is running
Pending Windows updates
Just like the setup software of ETS, the setup software of any tool may conflict with pending Windows updates (which actually also is setup software). To check whether there currently are any Windows updates being installed, see here.
Once the Windows updates are completed, try to install ETS again.
Active antivirus and/or a firewall software
Active antivirus and/or firewall software may block the ETS installation because of security reasons. In this case, it is recommended to temporarily deactivate the active antivirus and/or firewall software before trying to install ETS again. Moreover, it is recommended to add the ETS setup software in the 'white list' of the active antivirus and/or firewall software.
Wrong or incomplete .NET Framework
Since ETS5 v5.6.5 .NET framework 4.6.1 is required. Therefore, if an older version is installed or incomplete, it has to be updated. The update can be downloaded from here 
Another software installation is running
Another software installation is running while ETS is trying to be installed. Please wait until the other setup is completed and then try the installation again. If you do not see any other software being installed, then reboot your computer and try again.
If none of the above helps, then please follow the steps as described here.