How can I resolve the 'p--error' on my Casio HR-100RC?

To resolve the 'p--error' on your Casio HR-100RC, the first step is to check and replace the batteries. Ensure you are using fresh, high-quality alkaline batteries. If you are using the AC adapter, unplug it and try using only batteries, or vice-versa. Also, try removing the batteries for a few minutes and then reinserting them to reset the device.