Exported to Android by any chance? I think android prevents you from being able to assess res://. But I guess when you export a project the project folder goes away and everything is in the executable or .pck file. So the better method for saving files (reading/writing) is to use user://, which will always be there and is saved in the appdata folder. More info here.
It’s for windows, but thank you for the answer that I did’t know that