That is nothing to worry about.
ATK-Classical actually comprises 3 calculators: TremoloX, Brenner, and EMT, the last two essentially for historic reasons. When you open the New Calculator widget and select ATK-Classical, VNL checks all of these 3 calculators if they have potentials for the current configuration. If one of these 3 doesn't have parameters for the required elements, then this info message is shown, but hat doesn't mean that there are no parameters available in general.
In your case you can still choose the e.g. the suggested Tersoff potential (from the TremoloXCalculator) without any problem, and simply ignore that message.