It seems that PixInsight expects having the SITELONG and SITELAT FIT keyword in integer or sexagesimal format, but your software writes the data as
52d51m0.000s
where, I suppose, "d", "m" and "s" means degree, minutes and seconds, so the expression cannot be parsed (it is not a number) and PI returns you an error.
Anyway, I can guess that this error doesn't compromise your final integration except for the initial center of frame position used by plate solving tools like PhotometricColorCalibration.