3248190 - Items of type XXX must not be edited in this tenant | 3-System Landscape

• You are trying to use a Key User Extensibility app
• You are using a 3-System Landscape  
• You are getting the error message "Items of type XXX must not be edited in this tenant." or "Extensibility is not active. Use the customizing tenant instead."

SAP S/4HANA Cloud

You're trying to use a Development Tenant to use the Key User Extensibility. 

As you're using a 3-System Landscape, you have two tenants in your Development System:

• Customizing Tenant: Used for Business Configuration and Key User Extensibility.
• Development Tenant: Used for Developer Extensibility

Use the Customizing Tenant. If the "Add"/"New" button is greyed out in your Customizing Tenant, it means that you still need to activate the extensibility in this system. To activate the extensibility, please follow the steps below:

1. Go to "Extensibility Settings" app
2. Click on the "Key User Extensibility" button to set it to "ON" 

2283716 - Key User Application is not configured

SAP Help Portal - Extensibility Settings

Custom Fields, demo system, starter system, items of type Custom Field must not be edited in this tenant, extensibility, Extensibility is not active, customizing tenant  , KBA , BC-SRV-APS-EXT-FLD , Custom Fields , Problem