You're using Keil µVision, right? There's a bug in MDK-ARM that always copies "system_nrf52.c" from the latest software pack (8.29.0 at the moment I believe), even though you selected the version supporting your SDK. A solution suggested by the community is to replace the system_nrf52.c file found in the newest device family pack (ARM\PACK\NordicSemiconductor\nRF_DeviceFamilyPack\8.29.0\Device\Source) with the one found in an older version (try 8.17.0).
