Currently, uploading items to the new collections module with a list with over 1000 SKUs is impossible, since the process will timeout. It returns an unspecific error response: "Your file was not imported. Please try importing this file again."

1. Go to the new collections module
2. Select the tab "Import"
3. Use a sheet that has over 1000 SKUs (i.e. 300 products with 50 SKUs, each)
4. The import will generate a timeout.

Upload gradually using smaller sheets, the legacy CMS collections and/or the insert SKU to subcollection API https://developers.vtex.com/docs/api-reference/catalog-api#post-/api/catalog/pvt/subcollection/-subCollectionId-/stockkeepingunit