3291421 - Error 'Application can only be used in the Extensibility Development System' in Development System - Development tenant.

• Unable to open "Extensibility Inventory" app in development tenant.
• Getting error 'Application can only be used in the Extensibility Development System' while trying to open "Extensibility Inventory" app in development system - development tenant.
• Unable to open "Extensibility Inventory" app in parallel line system.

SAP S/4HANA Cloud Public Edition

• Using the wrong tenant/system to access "Key User Extensibility" apps.
• "Extensibility Inventory" app is disabled in parallel line system

Use development system - customizing tenant to access the "Extensibility Inventory" App.

Note : Key user extensibility is only available in the "Test/Quality" system in 2-system landscapes or in the "Development System - Customizing" tenant in 3-system landscapes. 

For more info, Refer the below documents

• 2283716 - Key User Application is not configured
• SAP Help Portal - Extensibility Settings
• SAP Help Portal - Transport Management Summary
• Extensibility Inventory | SAP Help Portal
• Key User Extensibility (Parallel Line)| SAP Help Portal
• 3342103 - Key User Extensibility apps are not available or read only - SAP S/4HANA Cloud Public Edition - SAP for Me

Key User Extensibility, Extensibility Tiles Error , Extensibility inventory, extensibility settings, customizing tenant, Application can only be used in the Extensibility Development System, Parallel line system, parallel customizing tenant (D.120) , parallel customizing tenant, Extensibility Inventory app is disabled. , KBA , BC-SRV-APS-EXT-SL , Manage and Import Software Collections , Problem