In Simplicity Studio ver.4, when building a new product, the definition file of the device is downloaded locally from the manufacturer's site, and it is necessary to be online at that time.
Actually, multiple definition files are added under the following path.
C:\SiliconLabs\SimplicityStudio\v4\offline

If you are offline, you will not be able to obtain this reference file, so you will not be able to handle it as expected. Please note that when selecting a new product, you need to build it online only for the first time.