make sure the Resource Group that you are trying to delete doesn't have any resources that have dependencies on resources in other RGs. For example, if you have a VM running in another resource group that is using an IP address assigned from a vnet/subnet in the RG you are trying to delete.
Also make sure there are no Resource Locks applied to the RG you are trying to delete or its underlying resources.
If this doesn't work, you may be best to raise a support request via the portal to have this resolved.
Hope this helps,
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