When you sign in to Lightroom Classic on your desktop, the following message is displayed:
Upgrade Required For Lightroom Mobile
Lightroom must be upgraded before it can continue syncing with Lightroom mobile.
Lightroom Classic 10.3 and earlier
Adobe has recently made some upgrades to Lightroom's back end cloud server, which enables syncing photos and edits between Lightroom ecosystem apps (mobile and desktop) and Lightroom Classic on your desktop.
If you are using Lightroom Classic 10.3 or earlier, update Lightroom to the latest version to continue syncing with Lightroom for mobile.