Error: External Tools - Add-in Assembly Not Found

Failed to initialize the add-in "TestFit integrations"

Cause

User doesn't have full admin rights to install add-in applications

Solution

1) Make sure Revit is closed.

2) Uninstall TestFit Addin for Revit

3) Delete C:\ProgramData\testfit_revit if it still exists.

4) Reinstall TestFit Addin for Revit

5) Relaunch TestFit

Note: You will need full admin rights to install the add-in on your machine.