I looked through the material for a pinout diagram for the Artemis Nano and could not find one. Or a table. It looks like Sparkfun has not done a great job of documenting the precise capabilities of the various pins on the Artemis Nano. Perhaps one of the developers can chime in here. Or you could post an issue on the Github site.
Checking the schematic, the pin labeled A14 has several alternative digital functions (see image below), so in principle it can be used as general digital I/O. However, the Arduino IDE has a lookup table specific to each board that specifies the allowed pin functions, and whoever created that table may not have provided a path for using the pin as digital I/O.
To address your problem, several pins on the right side of the schematic have labels beginning with “D” e.g. 4,6-13 and a couple of others, so try using one of those for digital I/O instead.
Some Arduinos do have “analog only” pins, and in those cases you simply have to be aware of the exceptions to the rule.
