AZ Copy Image - Cannot Find Target Subscription

- If the issue is to do with Azure CLI 2.0 in-particular, create an issue here at [Azure/azure-cli](https://github.com/Azure/azure-cli/issues)

### Related command
az image copy --source-resource-group rg-xxxxx --source-object-name vmxxxxxx
-image --target-location usxxxxxx--target-subscription "3abbd37a-xxxx-xxxx-xxxxxxxxxx" --target-resource-group "rg-xxxxx" --cleanup --verbose

### Extension name (the extension in question)
az copy image

### Description of issue (in as much detail as possible)

*I am not providing the exact subscription details for privacy reasons*

"I am trying to run this extension but continue to get a output: WARNING: Subscription '3abbd37a-xxx-xxx-xxxxxxxx' not recognized.
ERROR: Subscription '3abbd37a-xxxx-xxxx-xxxxxx' not found. Check the spelling and casing and try again."

The user running the command is a contributor at the subscription level on both the source and target. What is the problem?
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