Unable to access stm32_nucleo-f446ze using GDB server

[Sethunandan]

I am trying to flash and debug my .elf file with the help of openocd and gdb server in UBUNTU . After opening gdb prompt either using gdb-multiarch or arm-none-eabi-gdp and trying the command : target remote localhost:3333 , I am prompted with an error as shown:

I am a noob and have very less idea on GDB, openocd etc. Some references of GDB , telnet , openocd will also be appreciated