Issue:
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As a result of the current design in the process where the engineering company is releasing items to other receiving companies through the release product structure wizard if the receiving site has not been specified so the releasing site value will be copied to the receiving site.

however, in case of the site value doesn’t exist in the receiving company so the validation/accept process will be failed with the errors "Site XX does not exist." and "Check validation failed"

Suggestions:
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Implement a new logic to copy the receiving site in the module parameters or in the Product release policies.

Workaround:
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Specify a Template released product in the Engineering product category details assuming that this default item has default order settings (Inventory site) so this value will be also copied instead of the releasing site.