A formula in Quattro Pro returns "NA" instead of the expected value. This can happen for the simplest of formulas such as =A1+B1!
You will also see the circular reference () Indicator on the bottom toolbar in Quattro Pro.
This is caused by a circular reference. A circular reference happens when a formula in a cell relies on the cell that it is in to get the answer. A basic example is putting "=A1" into cell A1 in a sheet.
A more common situation is having a formula (Formula A) which points to another formula (which may point to yet another one... and so on), and one of these formulas points back to the cell containing Formula A.
Locate the formula(s) which refer back to their own cells and correct them. Using Tools > Auditing > Trace Precedents may help.
Also, look up "Tracing errors in formulas" under Help > Help Topics in Quattro Pro.