Hello
I want to know what is the command that I must add to the ble_uart code which exists in the SDK library to send hello word using the bleutooth in the nrf UART application .
Best regards.
Hello
I want to know what is the command that I must add to the ble_uart code which exists in the SDK library to send hello word using the bleutooth in the nrf UART application .
Best regards.
It tells you where the other declaration is:

You need to make sure that the two match.
the other declaration is in stdio.h , it is not in the same file :

Should I delete it?
no - they need to match.
Your implementation (definition) needs to match the declaration.
This is basic C programming stuff.
It seems that you really need to go back to the original, unmodified example and get that working - as Vidar Berg suggested.
yess I already do the build of the project without any modification and I hhave got this error