Unexpected exception while trying to download a Store App:
Further when checking SMP logs you will noticed a log Entry like:
Invalid 'Unc' package: '_unconfigured_package_416d8ce3-12b9-4b31-8818-3998413de734' (66d13ea3-6644-4e25-b175-98c93f60dbbe), location not specified, while it's mandatory.Parameter name: pkgItem
ITMS 8.8
Missing Software Library configuration
Configured Software Library for your ITMS environment as documented in the following: How to configure Software Library