I recommend resetting your bluetooth cache/data to do this follow the steps below.
1.tap on apps
2.press the three dots in the upper right corner and select system apps
3.scroll down to find Bluetooth tap it
4. Press storage
5. You will see two buttons "clear cache" and "clear data" if cache is faded and your not able to tap it press clear data and your pen should then pair
The reason for this is sometimes the Bluetooth cache can become courpted and need to be reset this happens on most device with Android OS installed on it.
Let me know if it works