3248190 - Items of type XXX must not be edited in this tenant | 3-System Landscape - SAP S/4HANA Cloud

You're trying to access a Key User Extensibility app in a 3-System Landscape but you're getting one of the error messages below:

• Items of type XXX must not be edited in this tenant.
• Extensibility is not active. Use the Customizing Tenant instead.

SAP S/4HANA Cloud

You're using the wrong tenant. There are two tenants in Development System:

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

You're using the Development Tenant for Key User Extensibility and this is not allowed.

OR

The extensibility is not active in your system yet.

In the SAP S/4HANA Cloud System, cloud starter edition or sales demo partner, extensibility for key users is deactivated by default.
Extensibility for key users can be activated.

Use the Customizing Tenant instead.

If the "Add"/"New" button is greyed out in Customizing Tenant, it means that it's necessary 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" 

OR 

If you're using the right tenant, you need to activate the extensibility. You can activate it by opening the Fiori application Extensibility Settings and follow the activation process.

KBA 2283716 - Key User Application is not configured 

SAP Help Portal - Extensibility Settings

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SAP S/4HANA Cloud Public Edition all versions ; SAP S/4HANA Cloud all versions