I know what the problem is.
I am using mbed-os version 5.1.4, 5.1.4 does not have ble/gap/Events.h.
Events.h exists in the master version and has a getRssi() function, but
When I change the version to master I get a build error.
To use the getRssi() function, I must use a higher version, but a build error occurs and I cannot do it.