You get the error because you are using the wrong GUID for this call.
The GUID 3030-68771 is a GUID for a game (in this case Assassin's Creed Odyssey, see https://www.giantbomb.com/assassins-creed-odyssey/3030-68771/) which you can use for the call /api/game/[guid]/.
The call /api/user_review/[guid]/ needs a GUID for a user review, e.g. for the stated game you could use 2200-30995 which would return the review https://www.giantbomb.com/assassins-creed-odyssey/3030-68771/user-reviews/2200-30995/).
Every resource has its own GUID with a different prefix (game = 3030-xxx, user_review = 2200-xxx, region = 3075-xxx, etc.)
Hope that helps.
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