If both Access Port Protection (APPROTECT) and Erase Protection (ERASEPROTECT) are enabled, access port 0 and the ERASEALL functionality are unavailable.
To unlock the device, it must have compatible firmware that provides a 32-bit non-zero KEY value to ERASEPROTECT.DISABLE. When both the debugger and firmware provide the same 32-bit non-zero KEY value to ERASEPROTECT.DISABLE, the device does a Control Access Port (CTRL-AP) erase all operation. The access port is re-enabled on the next reset once the erase sequence is done.