Do you know which minor revision are you on? The currently released firmware 2.2.1 is minor revision 0, and GPIO23 is still in the free pin list:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/defines.h#L444
If you are somehow on the 2.2.1 minor revision 1 branch: that’s an un-released firmware, where pin 23 is used for a button so is no longer in the free pin list.