First, uninstall Navisworks Freedom. (You should be able to reinstall it later)

The reason for this is that the SYNCHRO Pro Plugin for Navisworks will only install on Navisworks Manage and will NOT work on Navisworks Freedom. Since Navisworks Manage and Freedom probably share the same background libraries, files and folders from the SYNCHRO Plugin may not be getting installed in the proper places. To avoid the confusion, it is best to install the SYNCHRO Plugin when only Navisworks Manage is installed. 

Second, during the process of installing the SYNCHRO Plugin, you should see the following window/menu:

During the installation process of the plugin, it queries to see which version of Navisworks you already have on your laptop and automatically selects it. In the screen shot example above, this person had Navisworks Manage 2020 installed. Be sure that the version preselected during your installation matches the version of Navisworks installed on the laptop. If it is not correct, attempt to select the correct version before selecting the "Next" button.

Try following these steps: 

1. Uninstall the SYNCHRO Plugin for Navisworks
2. Uninstall Navisworks Freedom and any other version of Navisworks Manage
3. Install the SYNCHRO Plugin for Navisworks as Administrator
4. Confirm during the installation that the correct Navisworks version is selected
5. Complete the SYNCHRO Plugin installation
6. Confirm that the SYNCHRO Plugin Add-On is visible in the tool bar of Navisworks Manage

If all is successful, reinstall Navisworks Freedom and other versions of Navisworks Manage, if appropriate