You're trying to Import a Software Collection into your Customizing/Development Tenant but are not able to import.
SAP S/4HANA Cloud
This is the expected behavior. It's not necessary to Import anything into your Customizing or Development Tenant.
For a better undestanding I would like to explain the difference 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, you can build your own custom developments based on lifecycle-stable SAP objects, this is called Developer Extensibilty.
Customizing Tenant: This tenant allows you 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 you customize applications and their UIs. Using extensibility apps, you can create custom fields, adapt UIs, create data source extensions and implementation descriptions for specific business contexts to enhance predelivered applications that are extensible in order to adapt them to your business needs.
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