You can install the development version of the Adafruit nRF52 Boards platform by following these instructions:
https://github.com/adafruit/Adafruit_nRF52_Arduino#optional-core-development-adafruit-nrf52-bsp-via-git
Unfortunately, Arduino Pro IDE does not currently support boards platforms installed in the sketchbook, so you'll need to use Arduino IDE 1.8.14 instead.