After updating the Lightroom app on Windows, the error "Assertion failed" occurs when you start Lightroom.
Lightroom 5, Lightroom CC 2015/Lightroom 6, Lightroom Classic
Ensure that the My Pictures virtual folder points to the Pictures folder under your user account folder by doing the following:
If this solution doesn't fix your issue, contact Adobe Customer Support.
In Windows, a virtual folder called My Pictures represents the physical folder called Pictures that exists under your user account folder. Clicking My Pictures in your sidebar, for example, takes you to the Pictures folder under your user account folder: C:\Users\[your user name]\.
When the virtual My Pictures folder points to the physical Pictures folder that no longer exists, this error can occur. This error can be caused when the Pictures folder is no longer under your user account folder. It can also occur when the Windows registry no longer points to the appropriate Pictures folder.
The steps in this TechNote solve the issue in either case.