The installed app limit for the FR935 is supposed to be 32, but Garmin Express enforces a limit of 16. So sometimes you cannot add apps through GE. GCM works fine.
Environment
Garmin Express 5.5.0.0
OS: Windows 10
Re-creation
Install 17 or more apps on your FR935 (but less than 32). Attempt to add an app through GE. You will get an error message saying that you've reached your limit.
When you click on the "storage usage" icon under Manage Apps (3 platters?), the app will report something like "Apps: 31 of 16".