User unable to access sheets



I am attempting to give viewer-only access to an executive at my company. I have given him viewer access to both the workspace and the sheet he needs to view. His name and email address show in the list of contributors. However, when he attempts to access the sheet it says "You don't have access to this workspace. This workspace may have been removed or unshared", with a blue 'Request Access' button. When he clicks the button to request access, it prompts him to upgrade to a paid account. He is using a SSO if that makes a difference. Any ideas?