Do you have any Definitive Edition saves on Steam? If not, start a new game on PC, get through character creation and save (there is an auto save just moving a bit from the starting position), exit and make sure Steam syncs (usually it does so after exiting the game, but if not restarting the game should force Steam to sync). After that, try enabling Steam sync on the Switch again.
On PC, other than restarting the game, you can confirm what saves have been synced online with the program Steam Cloud File Manager Lite
); it can also download, delete or upload saves. You need to be logged in to the Steam client and have the game's AppID (435150). Hopefully just having a profile and save synced online will allow the Switch to link, but if not this may be useful in troubleshooting.
Alternately, you can log into Steam in a browser and use this link
(this will also let you download saves, but you would need to manually rename them, as Steam combines the path and name together for the online name).
On the Switch, try clearing the system cache: in the Home menu open System Settings, then select System, Formatting Options, Clear Cache and then the current user.
Note that this will also reset the system language and brightness setting.
Try changing your Steam password on PC, and then see if you can link from the Switch.