I've tried to remove the default stock and source, why I'm not allowed to do it?
You can't delete it because:
Any Sales Channels not assigned to a custom stock are assigned to the Default Stock.
- Reference: DevDocs Adding a New Stock
When you delete the stock, all assigned web sites are assigned to the Default Stock. We recommend reassigning websites to other stocks prior to deletion.
- Reference: DevDocs Deleting Stocks