Open Odin on your PC and connect your phone via a USB cable. The ID:COM box in Odin should turn blue, indicating a successful connection.
This usually happens if you forgot to perform a full system wipe before flashing, or if you flashed the wrong GApps package. Boot back into TWRP (Volume Up + Home + Power), perform a factory reset, and re-flash the ROM. samsung galaxy a5 2015 custom rom