The error message is giving you the answer here, access is not authorised.
As tThe Computer is outside of the domain and it more than likely does not have access to SSRS. If the other domain is part of the same forest, you'll need to allow the domain SSRS is on to be able to trust the other domain, and then add the users(s) or group(s) of the other domain permissions to do whatever they need on SSRS.
Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does :-P
Please always remember to encapsulate your code in IFCode Markup. For example [code=sql] [/code]
to read Jeffs Guide on how to post SQL questions, and get swift and helpful answers from the community