Unable to Interpret Value
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An example might be someting like:
Error found in Balance Sheet worksheet "Balance Sheet": Unable to interpret the value in the Balance Sheet Row: 3 Column: A. We were expecting a date.
Possibility #1 - The cell indicated has weirdness in it
Most often, this error occurs because the cell identified actually has something odd going on. It could be cell formatting, the presence of special characters, or perhaps (as in the example given above) the parser expected a date but instead ran into free text or a number.
Basically in all these cases, either the user made a manual mistake in an otherwise well-formatted document (which can be correctly easily in most cases), or the spreadsheet as a whole is in a layout / format that the parser doesn't understand. In the latter case, you'll need to make an educated guess about how much work it would take to "massage" the document into a positive format. For starters, take a look at our How To Format doc. In fact, if you think it would benefit the user to be able to do this kind of preparation on their own in the future (in most cases yes) then you can recommend that they have a look at that document and see if they can give it a try themselves. You'll want to make some suggestions about what changes to make, though.
... Or you could just do it for them, on the first go.
(Rare) Possibility #2 - The document has some deep weirdness in it
Very rarely, this error will occur because something else aside from the named cell happens to be confusing the parser. This could be something near the named cell, or it could be not-that-at-all. In the latter case, you might be stumped, so you would want to try copying / pasting (values only) into a new "clean" document. Often times that will remove whatever exotic formulae or cell formatting was buried in the document, and might resolve it.