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If the municipality that is selected does not appear in the irradiation table selected, the program applies the provisions of the UNI 10349:

For sites that are not included, you can calculate a correct irradiation identifying two locations of reference, which are present in the data tables, which are closer to the municipality that has been chosen.
The value of irradiation of the locality concerned is calculated as a weighted average of the values of the localities of reference weights with respect to latitude.
But we must pay attention to the fact that the algorithm in question performs a linear interpolation of irradiance values depending on the latitude. So a correct application of the algorithm requires that the chosen location must be within the latitude of the locality of reference, otherwise the results that are obtained may be unreliable. In some instances the irradiance values that are obtained can then also be negative.

To solve this problem you simply choose in the list of municipalities next to the one you choose, the reference municipalities so that one appears to the north and one to the south than that for which you seek the irradiation values.