Hi
When you use IF condition, it always returns either numeric values or String values you can't have both in one IF statement.
Try this :
if Left({SOP10201.SOPNUMBE},3)="INV" then "-na-" else "Days-"&
or
After placing this formula in your report, right click on it and go in format field --common tab--Display string --formula editor and give the follwing condition :
if Currentfieldvalue = 0 then "NA" else Totext(currentfieldvalue)