If you are getting the error bellow with Segger Embedded Studio versions (>= 6.20a) and nRF5 SDK you need to do the following three steps:
1. Delete SEGGER_RTT_Syscalls_SES.c (if it exists) from your project as shown below:
2. Update the library I/O to RTT in the project’s options as shows below:
3. Disable
RETARGET (if it is enabled) by setting RETARGET_ENABLED in sdk_config.h to 0
The reason to do these steps is that newer Segger Embedded Studio versions (>= 6.20a) has these functionality built-in so there is no need for the nRF5 SDK project to include it. This is explained here: https://wiki.segger.com/
Quoting from Segger documentation:
Note: Older projects might include SEGGER_RTT_Syscalls_SES.c or a similar file to override printf(). When using the SEGGER Runtime Library, this file should be removed from the project, or its content should be encapsulated with #if !defined(__SEGGER_RTL_SEGGER Embedded Studio provides RTT printf(); functionality automatically. To use the full RTT API, the RTT Target Package needs to be added to the Embedded Studio project: RTT in Embedded Studio