This article provides a solution for the issue where you cannot change the default metadata schema.
Steps to reproduce:
Ensure you do not have the metadata schema in
/dam/adminui-extension/metadataschema node in the project folder
/conf/project ID. The project folder is not checked for custom metadata schema.
If you do, remove the node from that path. New schemas should be created in
conf/global/dam/adminui-extension/metadataschema, the default for metadata schema. For more information, refer to the Metadata schemas, in the AEM as a Cloud Service user guide in Adobe Experience League.