Hello,
I do not have this hardware and I have no experiences with it, but…
It seems nrf52840 dk uses Segger J-Link . I do not know what is actual state , but Mbed Studio mainly works with DAP-Link/ST-Link and others external probes are experimental - Segger J-Link Support - Mbed Studio - Arm Mbed OS support forum
As second alternative you can try to replace your Segger J-Link with DAP-Link
- Daplink software released by arm-mbed for nrf52840 dk does not detect external target - Nordic Q&A - Nordic DevZone - Nordic DevZone (nordicsemi.com)
- Why use Mbed Daplink instead of on-board Segger J-Link ? - Question | Mbed
BR, Jan