Usually, when SolidWorks has been updated and the old versions of SolidWorks has been uninstalled. This error message tends to appear. It is nothing to worry as it can be fixed easily.
To fix this issue, we should check the default templates locations in System options:
Go to Tools > Options > Systems Options > Default Templates
There are 3 default templates: Part, Assemblies and Drawings.
SolidWorks will make use of these templates during automated file creation. Some examples would be when importing files, use of mirror part/assembly or saving bodies as part file.
These default templates need to point to the location where the templates exists. If you find that the templates no longer exists, you will need to click on the “…” and browse to a template that exists.
After updating these locations, the error should no longer appear.