3324929 - Not able to Import Collection into Customizing/Development Tenant (3SL) - SAP S/4HANA Cloud Public Edition

It's not possible to import a Software Collection into Customizing/Development Tenant.

SAP S/4HANA Cloud Public Edition, 3 System Landscape

This is the expected behavior. It's not necessary to Import anything into Customizing or Development Tenant.
For a better understanding, please be aware of the differences between the Development Tenant and the Customizing Tenant in terms of Extensibility:

Development Tenant: This tenant provides access to the SAP S/4HANA Cloud ABAP Environment. In this environment, it's possible to build custom developments based on lifecycle-stable SAP objects, this is called Developer Extensibility. 

Customizing Tenant: This tenant allows to create low-code custom developments in Key User applications, this is called Key User Extensibility. The Key User Extensibility is based on applications and features that help to customize applications and their UIs. Using extensibility apps, it's possible to create custom fields, adapt UIs, create data source extensions and implementation descriptions for specific business contexts to enhance pre-delivered applications that are extensible in order to adapt them to business needs. 

• SAP Help Portal | 3-System Landscape and Transport Management
• SAP Blog Post | Extensibility in the Development System of a 3-System Landscape

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