These are errors of "context," meaning that the formula may be correct, but is not working in the particular context where you have put it in the report design. Fixing these kinds of issues is difficult when the context is unseen. Can you post a screenshot of the report design, with some kind of comment as to where in the report you have put the Lookup expression?

What I can say is that the expected context is inside a text box within the "detail" section of a table that has been associated with the primary dataset. If this is in a grouped region or in a List or Matrix instead of a table, then the expression will need adjustment. But the method of adjusting will depend, again, on the context.