You need to share the folder as a network share before adding the library.  See this link:

https://support.sonos.com/en-us/article/add-your-music-library-to-sonos

Pay special attention this link within: https://support.sonos.com/en-us/article/share-your-windows-music-folder-with-sonos

Thanks for your reply. I had already read the information you refer to and configured my music folder as a shared folder accordingly, with "Sonos Wireless HiFi System (DELL-LAPTOP\Sonos)" as the user authorised to share. But I when I try to add the folder to the Sonos Library, I keep getting the message "Sonos cannot add the music folder. Reason: Computer"DELL-LAPTOP" not found. 

In the circumstances, maybe you should speak with Sonos support rather than fellow users? 

I do have to wonder if it is the hyphen in the name of the laptop, but then I’d imagine there would be hundreds, if not thousands of complaints about similar issues. 

Could be a non-private WiFi, or file & printer sharing disabled. Lots of potential issues here.

In File Explorer, if you go to Network, can you see your Sonos devices?

The error “Cannot find Dell laptop” occurs because the Sonos app cannot directly access music stored locally on your C-drive; the folder needs to be shared over the network. To fix this, navigate to your music folder (e.g., C:\Users\YourName\Music), right-click → Properties → Sharing tab → Advanced Sharing, and enable “Share this folder,” giving at least Read permissions to “Everyone” or your user account. Note the network path (like \\YourLaptopName\Music) and then open the Sonos app, go to Settings → Manage → Music Library → Add, and enter that network path, providing your Windows username and password if prompted. Ensure your Dell laptop and Era 100 speakers are on the same Wi-Fi network and that the firewall allows Sonos access. Once properly shared, Sonos should be able to detect your folder and play your music through the Era 100 stereo set.