I very recently tried to work with a normal CDF in Tableau and ran into the same issue. I do not believe Tableau has access to advanced mathematical functions and unsolvable integrals, such as the error function (Normal CDF). There may be a post in the Ideas section about adding some functionality for this. More to your point, Tableau is not designed to do any sort of statistical analysis, it is a visualization tool. If you need statistical analysis beyond the standard mean and standard deviation, you should consider doing your calculations elsewhere and importing the results.
On the bright side, there is a post in the Ideas section about adding functionality to R.
Hope this helps,
I would also be interested in having the ERF function available in Tableau. I know it's not meant to be a stats tools but given the amount of analysis capability it has it seems like expanding the statistics toolbox would be worthwhile.