841220 is, sadly, not a legal NACA airfoil designator.
There are three major families of procedurally-defined NACA airfoils -- four-digit like the 2412, five-digit like the 23012, and the 6-series like the 64A012.
It looks like this site only expects the four-digit form, and is mis-parsing anything other than that -- which is a pity, because one of the uses of a tool like this is to get a sense of /why/ the 23012 is similar CLmax to the 2412, but with lower moment and a more sudden stall. If nothing else, input validation is necessary!
There are three major families of procedurally-defined NACA airfoils -- four-digit like the 2412, five-digit like the 23012, and the 6-series like the 64A012.
It looks like this site only expects the four-digit form, and is mis-parsing anything other than that -- which is a pity, because one of the uses of a tool like this is to get a sense of /why/ the 23012 is similar CLmax to the 2412, but with lower moment and a more sudden stall. If nothing else, input validation is necessary!